On my last three projects I used Minwax spar urethane. (whether that's a real Polyurethane or not I don't know)

I polish them out using a $22 Walmart auto polisher and Dupont white fine compoound and then to any auto polish that's on sale (turtle wax, Dupont No. 7 etc.). My gloss comes out as bright or more so than the original varnish job. What it does is smooths out dust flecks. I varnish in my garage and no matter what I do (wet the floor etc) I still get some dust in a few hours before it sets up. This gives it that next level of smoothness. To answer your post - mine is as brigh and shiny as the original varnish.

Are you using an electric polisher? Huge difference between that and hand polishing. I polish it with wet polish for a while in a 2 ft sq. area and move on. Then I go back and hand rub off the dried sheen to a gloss.