danbob,

Maybe it's me but I think foam is often(not always) tough to tie in well. Some threads or too much pressure on the wraps cut through the foam. The foam may slide around the shank or where it's put in place. The fly, especially small ones, may not be very durable. Does that mean you got low-quality flies? Maybe. I think it's more likely for foam to break and then the wraps becoming undone, especially with smaller flies. This can happen because of trouts teeth or possibly removing the fly using hemostats. If the wraps are becoming undone and the foam is still in place, yes, the quality of the fly sux.

Allan