My door keeps coming up after fulling closing. I have tried adjusting the down force through it's full range until it bends the chain bar pushing down to so light it won't close at all. I have also tried backing off the down travel as far as the screw will go from all the way tight, trying every turn, and it still keeps coming up. the opener light does not flash. I have amber and green lights on my safety sensor. they work properly if I break the beam when door is coming down

SOURCE: Door will only close when held down

if you obstruct the photo eye does the light flash ?

SOURCE: Door will only close when held down

The problem is in the sensors at the bottom of the tracks. They both should have lights on them one orange, sending, and green, receiving. In almost all cases theres an indicator on each side to let you know that sensors are positively aligned. Check wiring.
The only other problem I have seen is if the opener has gotten damaged by a power surge or lightning. The safety sensor part of the computer board on the back can get damaged, and maybe a sensor or two but, everything else works ok.

SOURCE: Chamberlain door will not close, despite down force set to max

Problem solved, sort of-
My unit has self diagnostic capabilities: next to the remote sync red button, is a yellow light the indicates diagnostic readings by blinking. In my case the light was flashing 5 times after the door failed to close. This meant RPM sensor failure. I rechecked the sensor and "flywheel" cap and found that it was not completely secured. With the "flywheel" in the proper position, providing readings for the RPM sensor, the door completely closed.
But my yellow diagnostic light is still flashing 5 times. My door is operating correctly, but the diagnostic feedback is still not completely happy with my RPM sensor. So I will keep and eye on it.

SOURCE: lights dont come on

If you have a Craftsman/Liftmaster with a 3 function wall control:
use the lift button to turn the light off. push and hold the light button for @1min. until the light flashes. This should reset your operator to have the light turn on when you walk through the photo eyes.

SOURCE: Chamberlain Garage Door Opener will not close door

It is most likly your sensors You should not have to mess with your force settings but I can help you with your sensors assume you have checked to make sure there both looking at each other. First close the door so you can see them better. Tap each one of the sensors to see if any of the lights are flickering. if your then you need to tight them up and try again. Also when its dusk or night close your garage door and look at the lights see if one of the tracks is shaking sensor. If not check the wires make sure there is not staple or cuts in them. Then last thing you can check see if your door is hitting the back of the sensor mount it does happen. Also remove and ropes or handles you have fromt eh buttom of the door. That should take care of the problem. Make another post if your still having problems