Another possible issue is with varnish compatibility with epoxy.

If your epoxy surface is less than two weeks old the epoxy is still curing and can be incompatible with some varnishes.

If your epoxy is only a few days old it can cause even a compatible varnish to take longer than normal to cure.

The varnish has not cured properly if your sandpaper clogs up quickly. Cured varnish will turn to dust.

Check with the epoxy manufacturer to see which varnishes are compatible with their epoxy.

I don't use any solvents on epoxy or vanish except water. Most solvents are reclaimed and can contain contaminates which can cause varnishing problems.

Reducing the potential for problems is your best chance for success.