If the ththe second receiver you bought were the only one not to work I'd suspect a defective receiver, I got one from Airtronics that wouldn't bind right from new.

If the three receivers you tried at the shop are also all identical to the one that works, I think it's not likely that you ran across four defective receivers... and I don't know what's going on. If the three at the shop are a different model than your two, then they might be the incompatible FHSS three channel model.