If there's any one culprit that creates more havoc on builders it's blush.

Blush on the surface will prevent varnish from drying. Epoxy is actually inert once it is fully cured and any varnish will work fine: HOWEVER if you don't wash the surface with some soap and water to remove any blush, it will highly interfere with the drying of varnish. Like Rob, I too had it happen and had to remove varnish and start over. When people remove the varnish they usually accidentally clean the blush off too. After talking to epoxy manufactures and research, I learned that the culprit is blush. Plus epoxy should be left to harden for weeks before varnishing. When it doubt, give it another week and scrub that surface with a scouring pad and a little soap and water and wipe up with paper towels - then do it again.