What you're describing really doesn't sound consistent with the way the resin should perform. I appreciate that you are a painter and so you are confident with your mixing but no matter what your level of experience it's always possible to make mistakes (we make them occasionally and we work with these products every day). If you are applying subsequent layers onto tacky resin then the new layer should crosslink properly with previous layer and the bond between the layers should be better than you can achieve by allowing the resin to cure and then keying it back.

The only two logical explanations are that the resin has either cured too far to crosslink (therefore you need to apply your next layer sooner) or there is some surface contamination on the previous layer which is interfering with the next layer's ability to bond to it - this is likely to be caused by insufficiently thorough mixing or inaccurate mix ratio. I would suggest triple-checking your mix calculations (maybe you could share them with us), making very sure you're mixing thoroughly (I suggest using a double-potting technique) and also reducing the time you wait before applying one coat onto the next. Correctly followed, I would be staggered if this does not resolve the issue.