Sorry to hear about your predicament. This is a big reason that I do not guarantee items that the customer provides. Additionally if you do items on customer supplied items I would recommend that you have the person provide several extra items that you can apply your decorations to and wash them several times to check out the durability.
In your situation at this point I would use one of the jerseys as a test as follows:
1) Attempt to completely remove the number from the jersey.
2) Wash and dry the jersey 3 times according to the wash/dry directions then re-apply the number to the jersey.
3) Wait at least 24 hours for the adhesives to completely cure then wash and dry an additional 3 times to check the durability.
Doing this will 1; remove excess dye and sizing from the garment that could be effecting the adhesion of the glue. 2; allow you to verify that you are getting good adhesion on a sample prior to running them all over again.
I would also run a test on your heat press to insure that you are getting the proper temperature, if you press is cool you will not get good adhesion.
Try some ThermoFlex Xtra sometime down the road, we use this for all nylon & synthetics and have never had a problem with any adhesion, even on leather.
http://www.beacongraphics.com/heat_t...flex-xtra.html