Windows autentication PROBLEMS

[naija naija]

HI
I was using Windows Authentication before in my LOCALHOST with
BuiltIN/Administrator(And i was getting DOMAIN\USERNAME and it was
working now implemented the Windows Auth in a Production server and I’m
getting the Context.User.Identity.Name. too
But when I use IsinRole for any domain\group its doesn’t work!
Can you tell me anything that ‘m not doing well!

Thanks