One factor you didn't mention is how long the varnish has been on and how much UV the varnish/epoxy mix has been exposed to. I have seen lack of adhesion problems between varnish and epoxy that I attribute to UV deterioration of the epoxy at the interface between the epoxy and the varnish. This problem usually makes itself know 3-5 years after the application or immediately after stripping of the varnish and reapplication over an older epoxy.

There is also no doubt that getting oil based paint products to bond to a plastic as inert as epoxy, even new epoxy can be challenging and at times frustrating. That being said, I've had good luck with Z-spar's Captains Varnish over MAS and WEST epoxies over the last ten years or more.

If you think the problem might be UV related, you might consider switching to Petite's Bak-View phenolic resin based varnish. For a conventional varnish, it is by far and away the longest lasting in extreme UV. It dries too fast for brushing, but we have found it sprays nicely. We have used this over epoxy coated cold molded constructions in the past with excellent results.