It sounds like you might need to adjust your "up force", as it may be set too low. However, to be safe you should never work on these things or make these adjustments if you don't have the original instructions. There is a very strict and ordered procedure to follow in order to have these work correctly and safely. Other issues to be checked first: Are your rollers and/or track correctly lubricated? Did your track get bent or twisted or do you have a damaged roller or other obstruction? Are your springs adjusted correctly (you need the instructions to know this)? Does the door open and close manually freely and without catching or too much resistance?
You need to answer ALL of these questions before you set out to recalibrate the motor stop points and/or force adjustments. Only then can you step by step follow the instructions to adjust the motor correctly. If you can't find the instructions then start hunting, maybe someone who sells them will give you a copy or let you photocopy them, or go to google.com and search for them, almost everything is on the web somewhere.
Garage doors can be extremely dangerous if they aren't maintained or adjusted correctly.