Replaced faucet cartridge but can't seat stem properly

Cartage(s) may be either backwards or switched with each other.
Other possibilities the spring got out of place.

Make sure the water to the tap set is turned off. Remove the hot plug button, use a small knife to pry it out of the top of handle. Remove the screw inside the handle that holds the handle to the cartridge. Use an adjustable wrench to unscrew the tap handle from the base. Once removed, separate the cartridge from the shell (the unscrew from each other). Use the appropriate size and type of screw driver to remove the screw holding in the flat washer. Using a small dull needle or flat screwdriver remove the "O" ring from the valve stem Replace the o ring by just slipping it on. Make sure you clean all areas of the stem and the stem cartridge of old grease and dirt etc. Use new clean valve stem grease on the O ring before re installing into the cartridge. Insert the new washer onto bottom of valve stem, Screw on. While you have it apart look down inside the tap to inspect the valve seat. If its chipped, broken, rusted etc you MUST replace it, if you do not any new parts will sot stop the leak. Valve seat MUST be nice and flat and clean. To remove the seats most taps will require a 3/8" Allan wrench. Push the wrench into the slot of the seat, turn C-Cw to remove, CW to replace. These parts are cheap. If you need to replace the seat, take it with you to H Depot so they can give you the correct one. NOTE: Its best to do the cold at the same time, saves you the trouble of doing it again later. Same applies to the cartridges. You can replace the entire cartridge instead of rebuilding them at only a few bucks each, its much faster and easier.

Either you didn't seat the o rings properly or they are the wrong size or the cartridge needs replacing

If your faucet is dripping out of the spout. This is how to repair it if you have a faucet with this type of cartridge system

For details see diagram attached.
In the order you see. Valve nut, o-ring (may or may not be used in your valve) Valve Cartridge, Cup Washer and Spring.

Turn off the water to the valve at the water supply under the sink if you have shut-off valves there. Otherwise you will have to turn off the water to the whole house. Open some of the other faucets in the house if you have to turn the whole house water supply off at the water meter valve to reduce the line pressure. Open the handles of the faucet you are working on as well after you turn of the water either way.

If it has two handles there should be a set screw that holds the handle on. You will need the appropriate allen wrench for the set screw. Loosen that and wiggle the handle off the valve stem carefully. There is a brass nut that holds the valve cartridge in the valve body. Using a small pair of channel lock pliers or the appropriate box wrench. Loosen and remove that brass nut. Once that is off, grasp the valve stem with the channel locks and gently wiggle the valve as you pull upward. There will be some suction that wants to hold it in so this may take some finessing to loosen and remove. Be firm but gentile. Make sure you pay attention to the position the cartridge is in so you can place the cartridge back in the same position it was when you removed it. Otherwise, the handle will not be in the proper position when you put it back on.
Once you have the cartridge out. There should be a small black cup washer that sits in a hole on top of a spring at the bottom of the inside of the valve body. You should replace both the spring and cup washer.
You can find them at your local hardware store or big box DIY store.
I use a real small screwdriver to lift the washer and spring out.
When you go to place the new cup washer and spring back in place.
Place the spring in the whole with the larger side down. Then, to place the cup washer back. Place the cup washer on the small screwdriver and hold it with your index finger and lower it to the whole over the spring. Once you have the cup washer over the spring. You have to get the wider bottom part of the washer into the hole. Holding the washer down with your index finger. Use the small screwdriver tip to push the side of the washer into the hole working your way around the washer. Pressing the sides of the washer inward so the bottom edge pops into the hole over the spring. Once you fell the washer pop in and seat. Push straight down to get it fully placed. Then reverse the procedure you used above to remove the cartridge. Paying careful attention to the position the cartridge was in when you removed it and replacing it in the exact same position. There are o-rings or an o-ring around the cartridge so you'll have to wiggle the cartridge and press down to get it to seat level. Place the nut back over the cartridge and tighten down firmly. Replace the handle and tighten the set screw.

use product and let it sit for a while and it should clean its self and release

You can go to the link below to get a parts breakdown of your style of Delta. The seats and springs are recessed into the valve body behind the RP1991 stem. The flat part of the stem rotates back and forth across surface of the rubber seat, so in time the rubber surface wears flat and the spring can no longer push it firmly against the stem.and then the faucet drips.


http://www.plumbingpartsdepot.com/delta1600.pdf

#1 THE FAT PART OF YOUR SPRING GOES INSIDE THE FAUCET ,AND SIT ON THE VALVE BODY(SEAT)
#2 THE LITTLE BLACK WASHER SIT ON TOP OF YOUR SPRING
#3 PUSH THE STEMS IN THE CORRECT SIDES HOT ON LEFT.CAUTION THERE IS USUALLY NOTCHES ON THE SIDES OF THE STEMS TO HELP YOU ALIGN THEM IN THE VALVE BODY,
4 PUT THE NUT BACK ON ,TIGHEN SLIGHTLY,TURN ON THE WATER TO CHECK ,IF OK RE INSTALL YOUE HANDLES

Remove it and make sure that the cup washer and spring are seated propoperly. The large end of the cone shaped spring sits on the bottom of the housing seat and cone washer slips over the small end of the spring. Use the shank of small screwdriver to guide the washer and spring by placing them on the shake in the proper order and then putting the shank into the housaing and sliding the washer and spring.
You may also have to disconnect the supply tube that connects to the faucet under your sink. This is to relieve the pressure within the valve housing which can cause the washer to become mis-aligned when installing the cartridge
. Also check cartridge for defects and make sure you have it install in the correct position i.e you may have to remove it rotate it 180 degrees re-install it and move the limit stop (if it has one, depends on year) inserted in the top to the correct side for propoer operation direction of the handle after you reverse it.
Hope this helps, sincerly T. Wood, C&F Enterprises, A Plumbing Contractor