As others have suggested, the suspicion falls on the tensioning/roller slide mechanism.
No way should any belt be that oversize that the tensioner can't take up the slack. If it is the belts that are way over size, please take 'em back. The manufacturer needs to know they've got a problem!! If you only bought 1/2 doz. can you imagine how many dozens (thousands?) are potentially out there?
In fact, cut to the chase; phone the Manufacturer and let them know about the issue. If they're reputable, they'll just send you new ones (maybe some freebies for your trouble)>