Dead Frogspawn

For the second year running our frogspawn is not developing into tadpoles.  We have a medium size pond, depth about 4 feet.  We have lots of frogs laying lots of frogspawn (this year early March).  We created our pond about 12 years ago and have had successful frogspawn growing into froglets and small frogs.  We have frogs overwintering in our garden.  The frogspawn looks as if it is developing and then goes mushy.  We even took some frogspawn out of the pond and placed in tanks so that we could 'rear' them and replace back into the pond, but these have not developed either.  Is this a problem with the frogs?