I have never used that particular brand of head cement, i prefer either epoxy or superglue-depending on the fly im tying. However are you adding more than one coat? if so then im sure your aware you need to wait until its fully dried before adding another.

Finally if that doesn't work is it possible its too thick? often that's the reason for it not fully setting, there are thinners available to make it less viscous and improve its coating ability.

If the product still flakes off then i would suggest returning it to the place your purchased it from, it could have been stored in hot conditions then allowed to cool very quickly-something i have heard to ruin head cement before.