DIAGNOSIS If the sensors are fine, the opener's force sensitivity is set too low. It thinks it's hitting an obstruction (like a person) and automatically reverses as a safety feature.
FIX You need to adjust the force settings. On the back of the wall control console, you'll find two small dials, one for 'Up Force' and one for 'Down Force'. 1) For a door that reverses on the way up, turn the 'Up Force' dial clockwise one notch. 2) Try running the door. 3) If it still reverses, turn it one more notch. Repeat until it opens fully. Do the same with the 'Down Force' dial if it reverses on closing. Increase the force in small increments.
IF THAT DIDN'T WORK The door itself might be out of balance or have a stiff roller, making it harder to move. Disconnect the door from the opener (pull the red cord) and lift it manually about halfway. Let go. It should stay in place. If it slides down, the springs are weak and the opener is working too hard. Call a pro for spring adjustment—that's dangerous to DIY.
HEADS UP After increasing the force, test the safety reverse. Place a 2x4 on the floor under the center of the door and close it. The door must reverse within 2 seconds of hitting the board. If it doesn't, your force is now too high and unsafe.