Beater does not turn

If it is a ksm103 there should be a spring on the shaft, it should look similar to this: http://www.mendingshed.com/kacentershaft.html

When adjusting the screw you should be turning it out, which will make the beater closer to the bottom of the bowl, when properly adjusted you should be able to place a dime in the bowl, and the flat beater should just move it around barely. that would be about 1/16" in clearance.

Use a small amount of vegetable oil to lubricate the beater shaft and
the beater. Slightly tap the beater up and sideways with a rubber mallet
until you can move the beater easily along the shaft.

Make sure your Beater can slip in and out of the Beater Shaft
easily. Do not force the beater onto the Shaft if the shaft is bent.
If it cannot be removed easily by hand, use a soft rubber mallet
(not a steel hammer) to slightly tap the beater so it can be moved
up and down the shaft or from side to side. Then you can easily
turn the beater left and remove it from the shaft.

From Kitchenaid Manual:
To Attach Flat Beater, Wire Whip, or Dough Hook
1. Turn speed control to OFF.
2. Unplug stand mixer or disconnect power.
3. Tilt motor head back.
4. Slip beater onto beater shaft and press upward
as far as possible.
5. Turn beater to right, hooking beater over pin on shaft.
6. Plug into a grounded 3 prong outlet.

To Remove Flat Beater, Wire Whip or Dough Hook
1. Turn speed control to OFF.
2. Unplug stand mixer or disconnect power.
3. Tilt motor head back.
4. Press beater upward as far as possible and turn left.
5. Pull beater from beater shaft

Make sure your Beater can slip in and out of the Beater Shaft

easily. Do not force the beater onto the Shaft if the shaft is bent.

If it cannot be removed easily by hand, use a soft rubber mallet

(not a steel hammer) to slightly tap the beater so it can be moved

up and down the shaft or from side to side. Then you can easily

turn the beater left and remove it from the shaft.

From Kitchenaid Manual:

To Attach Flat Beater, Wire Whip, or Dough Hook

1. Turn speed control to OFF.

2. Unplug stand mixer or disconnect power.

3. Tilt motor head back.

4. Slip beater onto beater shaft and press upward

as far as possible.

5. Turn beater to right, hooking beater over pin on shaft.

6. Plug into a grounded 3 prong outlet.

To Remove Flat Beater, Wire Whip or Dough Hook

1. Turn speed control to OFF.

2. Unplug stand mixer or disconnect power.

3. Tilt motor head back.

4. Press beater upward as far as possible and turn left.
5. Pull beater from beater shaft.

Make sure your Beater can slip in and out of the Beater Shaft
easily. Do not force the beater onto the Shaft if the shaft is bent.
If it cannot be removed easily by hand, use a soft rubber mallet
(not a steel hammer) to slightly tap the beater so it can be moved
up and down the shaft or from side to side. Then you can easily
turn the beater left and remove it from the shaft.

From Kitchenaid Manual:
To Attach Flat Beater, Wire Whip, or Dough Hook
1. Turn speed control to OFF.
2. Unplug stand mixer or disconnect power.
3. Tilt motor head back.
4. Slip beater onto beater shaft and press upward
as far as possible.
5. Turn beater to right, hooking beater over pin on shaft.
6. Plug into a grounded 3 prong outlet.

To Remove Flat Beater, Wire Whip or Dough Hook
1. Turn speed control to OFF.
2. Unplug stand mixer or disconnect power.
3. Tilt motor head back.
4. Press beater upward as far as possible and turn left.
5. Pull beater from beater shaft

Get a copy of the Repair Part List for your model at Kitchenaid.com
Download the following Kitchenaid Service Manual:
http://www.kitchenaidparts.be/centers/servicemanual/mixer.pdf

It covers several models including yours.
1. Check/Adjust the Flat Beater to Bowl Clearance:

Turn screw (A) SLIGHTLY counter clockwise (left) to raise the flat

beater or clockwise (right) to lower the flat beater
2. Check the Beater Shaft (Agitator Shaft) making sure the Retaining ring
is flushed with the surface of the Planetary Assembly
Constant Pulling and Removal of the beaters can cause the Agitator shaft to move
3. If the above two still don't solve your problem, follow the service manual
directions to disassemble the Planetary Assembly and check for loose screws

1. Unplug stand mixer.
2. Place bowl lift handle in down position.
3. Attach flat beater.
4. Adjust so flat beater just clears bottom surface of bowl
when in lifted position by turning screw
(A) Counterclockwise to lower the bowl.
5. Just a slight turn 90 degrees in either direction.
Place bowl lift handle in up position to check clearance.
6. Repeat Steps 4 and 5 if necessary.
I hope I helped you.

You have probably stripped a gear in the gear housing. Here is the breakdown of the gearbox:

http://shared.whirlpoolcorp.com/assets/pdfs/literature/Repair%20Part%20List%20-%208212010.pdf

The newer "more improved" mixers use plastic gears and they can't take the beating, no pun intended, and fail.

The good news is that the most likely gear to fail is cheap. Here it is on amazon.com:

http://www.amazon.com/KitchenAid-W10112253-Worm-Gear/dp/B0006HLAVA

http://www.misterfixit.com

JOE:You don’t say if you have a ‘tilt-head’or a ‘bowl-lift' mixer, but you can easily download a PDF version of your Owners Manual from the KA website (http://www.kitchenaid.com/flash.cmd?/#/page/home).Click on Customer Care>Locate Manuals& Guides> and enter the 6 character model number (in your case, use KSM90P)and select the last file listed that appear under Owners Manuals:KSM90PSWW0.pdf (2930.74 KB)and save that file to your PC.You’ll need to have Adobe Reader to be able to open it. Turn to Page 11 and you will find the instructions for adjusting the mixer head height.I have extracted the instructions below:
Beater to Bowl Clearance
Your mixer is adjusted at the factory so that the flat beaterjust clears the bottom of the bowl. If for any reason, the flat beater hits thebottom of the bowl or is too far away from the bowl, clearance

Tilt-Head models
• Unplug mixer.
• Lift motor head.

• Turn screw (A) SLIGHTLY counter clockwise (left) to raise flat beater or clockwise (right) to lower flat beater.
• Make adjustment with flat beater, so it just clears surface of bowl.
If you over adjust the screw, the bowl lock lever may not lock into place.


Bowl-Lift models
• Unplug mixer.
• Place bowl lift handle in down position.

• Turn screw (B) SLIGHTLY counter clockwise (left) to raise flat beater and clockwise (right) to lower flat beater.
•Make adjustments with flat beater, so it just clears surface of bowl.

I hope this helps! –Howard, Burke, VA
If this solution has helped you, please rate it, thanks!.hslincoln

you most likely stripped the plastic gear (Worm gear???).  This gear is designed to fail to prevent the motor from burning out under EXTREME loads (i.e. when a metal piece falls into the bowl). The plastic gear is much cheaper than a new motor.  Look around on this site to see how to get at/repair the gear.  I've found many parts on eBay.com and also PartSelect.com.