the most likely cause for this is the upforce adjustment screw needs to be adjusted giving more pull upward however this is a sign that the problem could be the door and not the opener to cheach this disconnect the door from the opener with the quick release rope and try to raise the door by hand if this is very difficult cheack to be sure the spring or springs on the door are not broke or a cable off of the cable drums if all of this is ok but the door feals "heavy" then the springs have likely stretched and need to be tightened if torsion springs or replaced if extension springs.note extension springs are the type on each side of the door that stretch when the door is down and retract when the door is up torsion is over the top of the door or sometimes in the rear of the tracks at the top the springs wind tighter when the door is down therefore giving tension and giving help to you when raising the door then the weighjt of the door helps to rewind the spring whjen it is lowered these problems must be fixed as soon as possible to prevent completely burning up the motor on the opener

I had a similar problem. I found that the two sections of the T-shaped bar that the carrier rides on become slightly mis-aligned. I could give it a good smack with a hammer and it would work again for weeks or months. The problem was worst in cold weather. I recently replaced my old unit with a new Craftsman and they have redesigned that bar.
Also, make sure all bolts and nuts are tight throughout the system and that the chain is properly adjusted and that all hinges and rollers on the door are lubricated and working smoothly.

[This message has been edited by rpxlpx (edited January 03, 2001).]