Loosen the adjuster three turns counterclockwise if the door raised without.
Tighten garage door belt.
But basically assuming the belt is attached with enough slack removed the tensioner bolt should be able to taske up the rest of the slack once tightened.
This is a very simple job to do yourself in fact all you need to do is follow this simple 6 step guide.
The manual does not indicate how to solve this problem.
The specific procedure should be in the manual for your unit.
The only thing you need to do with a belt driven opener is to adjust the belt occasionally.
All you need to get started is a step ladder a 7 16 inch wrench and a flat head screwdriver.
Tighten the adjuster three full turns clockwise to add tension if the door closed and did not remain open halfway.
Brand new chamberlain garage door opener with excessive slack in belt drive.
1 close the garage door.