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

I think the adjustment mode is on.

For example, that's from Craftsman manual:

""
Setting the Force
The force setting button is located on the left panel of the motor unit. The force setting measures the amount of force required to open and close the door.
1. Locate the purple button on the left panel of the motor unit (Figure 1).
2. Push the purple button twice to enter the opener into Force Adjustment Mode (Figure 2). The LED (Indicator Light) will flash quickly.
3. Push the remote control or control console (Figure 3). The door will travel to the DOWN (close) position.

""

Actually, you should just read carefully your garage door opener unit's manual.

Craftsman Garage Door Opener Remote

Check the photo eye, if it is mis alligned or broken it would cause this type of problem.

it needs adjustment, there are usually an adjustment knobs on the unit it self, (check from the owners manual)...

Yes, reread the directions travel needs to be set for both up and down, and MUCH MORE IMPORTANTLY the safety settings must be set. This allows the door to close on a foot and not cause damage....VERY IMPORTANT to set correctly....

Had similar problem. Re-program travel and force limits. Make sure that travel limit (down) does not go too far (thus excessive force). Apparently this system has a force sensor that senses if there is something in the way of the door. This is different from the safety sensor "eyes". The problem in your setup is that you have programmed the door to travel down too much and he system looses its ability to use force sensor. Adjust travel limit (down) to a little less travel and you will be fine.

YOU NEED TO ADJUST THE DOWN FORCE.

YOU WANT TO TURN IT COUNTER CLOCKWISE TO THE STOP (THIS ADJUSTMENT IS ONLY ABOUT 3/4 TURN FROM MIN. TO MAX.). THEN TRY THE DOOR IF IT REVERSES TURN IT ABOUT 1/16TH TURN CLOCKWISE. CONTINUE THIS VERY MINOR INCRAMENT ADJUSTMENT UNTIL THE DOOR CLOSES WITHOUT REVERSING. THEN GO 1/16TH TURN FURTHER.

YOU SHOULD BE ABLE TO STOP THE DOOR AND REVERSE IT BY GRABBING THE BOTTOM OF THE DOOR AS IT GOES DOWN AND LIFTING SLIGHTLY. IF YOU CAN'T GET THE DOOR TO BOTH CLOSE AND REVERSE WITH MODERATE PRESSURE YOU MAY NEED ADJUSTMENT TO THE DOOR / SPRING / ETC.
- I DO NOT RECOMMEND ADJUSTING THE DOOR YOURSELF - CALL IN A PRO FOR THAT.

IF THE ADJUSTMENT IS SUCCESFUL ADJUST THE OPEN SENSITIVITY / FORCE IN THE SAME MANNER.

HOPE THIS HELPS

Are you turning travel screw, or force screw? If travel, One is for how far up and other is for how far down. Insert screwdriver into UP adjustement and turn it opposite direction of arrow. Try 2-3 turns to start. Yu may have to turn back CW. If while adjusting door just stops, go have a cup of coffee. You overheated motor. Give it 15 minutes to cool down.

It sounds like you have a broken door spring. Look closely - some breaks are hard to see.

THE TRAVEL ADJUSTMENT IS ON THE SIDE.
YOU MAY BE ADJUSTING THE SENSITIVITY (TRAVEL LIMIT HAS NO MAX ADJ.)
BE SURE TO RESET THE SENSITIVITY ADJUSTMENT TO ABOUT 1/4

HOPE THIS HELPS

You can overridride by pushing and hold wall button. If the door goes down and reverses up about a foot from the floor it could be a force setting issue on the opener or an issue with the door. First, pull the handle on the opener and operate the door manually a few times to make sure it opens and closes freely. You should be able to open and close the door with one hand or there is a problem with the door. If you determine that the door is ok then look at the force setting knobs. They are located on the back and have numbers from 1-10. These should be set on about 4-5 for the upforce and the downforce. I hope this helps.