If the T-Shirts were laundered with fabric softener or dried with a fabric softener/anti cling dryer sheet that will keep the fusible from fusing. Rewash with no additives other than detergent. Dry with no dryer sheets.
To test to see if your interfacing is defective, try fusing it to a scrap of regular fabric from your stash that has been prewashed (factory sizings also impede the fusing glue). I would recommend testing prior to rewashing all the t-shirts.