Maybe with some pics and a little more info we will be able to give the correct answers, but meanwhile...
Did you use gloss varnish?
liquid or spray varnish?
Did you let ENOUGH time for your paint to dry (specially true for washes and metallics)?
Do your paints and varnish use the same medium (acrilic or enamel)?
Maybe did you used another medium for your shading (gouache)?
In my own experience, gloss varnish may give the IMPRESSION that some of your shading and higlighting are lost, due to the strong light reflections.
Also, if you usually paint under ARTIFICIAL LIGHT, and move your work to DAYLIGHT, most of the time there will be some differences.
That's natural, and that's why I use daylight bulbs, but as we usually play with artificial light anyway, that's not one of my major concerns.
Hope you give us some more details and maybe a few pics so we can help you.
Hope that helps.