Surface contamination causes fisheyes to form in a varnish finish. Fisheyes are small spots that the varnish will not stick to. They can range in size from 1/8" to over 1". Poor surface preparation at the start is the most likely cause of surface contamination, but it's also possible that a surface contaminant wasn't removed before you sanded the surface prior to top-coating.
To prevent the fisheye effect in your varnish finish, always follow all surface preparation instructions carefully, and never substitute the recommended surface prep solvents.
If you do get fisheyes, you'll need to remove the coating from the affected area, redo the surface prep, and reapply the varnish.