DIAGNOSIS The travel limits are set too tightly, so the opener thinks the door has hit an obstruction the moment it starts moving down. The force settings might also be too low.
FIX You need to adjust the down travel limit. On the side of the motor unit, find the two square adjustment knobs—one for UP limit, one for DOWN limit. With the door fully open, press the down button. As soon as the door starts to reverse, turn the DOWN limit knob clockwise (usually a quarter turn) to increase the travel distance. Test close again. Repeat in small increments until the door closes fully and seals on the ground without reversing.
IF THAT DIDN'T WORK Next, adjust the down force. The force adjustment screws are next to the limit knobs. Turn the DOWN force screw clockwise slightly (increasing the force). Don't go more than a quarter turn at a time. The door should stop and reverse if it hits more than about 15 lbs of resistance, so you want it just strong enough to close against normal weather sealing.
HEADS UP After any limit adjustment, test the safety reversal. Place a 2x4 or a roll of paper towels on the floor under the center of the door. Close the door. It must reverse immediately upon contacting the object. If it doesn't, your force is set too high and it's unsafe.