New garage door opener belt sags when opening
I had the same issue on a brand new opener I just installed. The non-pulling side of the belt was loose and floppy when opening. Turns out I just didn't follow the instructions for tensioning the belt properly. 

Tensioning instructions right here: http://support.chamberlain.com/articles/How_To/How-do-I-check-and-adjust-the-belt-drive-tension-1484145517129

Once I made sure that the tensioning nut was tightened until the spring was compressed to 1-1/4 inches as instructed, the flop in the belt went away.