PU sealers are not the strongest, when i used to bond punto ones on we would only use windscreen sealer for bonded screens as its very strong, start again and clean all the old stuff off, ruff up the areas that will be bonded, refit and tape down (tightly) overnight it wont fall off again, i remember the punto spoilers were always falling off and people nicking them but i never got a comeback! good luck Ant! all sealers need to be kept above a certain temp or they wont cure especialy in this weather