your down force, or the revers mechanism are out of adjustment. there are knobs on the unit that are marked upforce, downforce and not sure what the reverse is called, but that would be my first clue. Aother thing to look at would be the sensors at the bottom of the door. There should be one on either side if yours is a fairly recnet unit, not sure what year they started using them, one has an led on it that lights up when it sees the other unit. If no light is on it, try to realign them until it lights up, if it doesnt check the wiring. When you press and hold the button you override that safety mechanism. If in doubt call a profesional to inspect and adjust your unit.

Thank you so much. You know, the sensors never have been installed. Maybe that is the problem. I didn't realize that it was mandatory for them to be installed. I will try what you suggested and then see if that works. If not, I'll get the sensors installed. Thanks for your time!!