This malfunction is common and it almost always has to do with the opener units limit assembly or switches telling opener when to stop. Some openers have the up and down limits inside the motor head and some are external mechanical switches. Another possible reason can be the opener pressure is set to low and while not common it is possible. There also may be something on the garage floor that the door is hitting and activating all openers safety reverse to keep from injuring people or damaging cars. So first start with the easy one and check to see if any objects are in the doorway. Also if someone tampers with the openers limit settings trying to fix a problem themselves can knock adjustments out of wack.

simple factors…

If all the simple factors are not the problem than there is a bad down limit switch. Or on some brands the limit assembly may have a loose wire or the gears are not touching. Some manufactures will put a movable notch or clip on the chain or belt to set the up and down limits and sometimes these clips can break off and totally mess up the limits on an opener unit. Bad limit switches or the systems that control them can always be repaired so there is no reason to replace the opener unit unless it has some age to it and its just time for a new one. New garage door openers have some pretty cool features and if you are keep up with new technology than you will appreciate some of the new digital options they offer.