What kind of hops are you using? One possible issue could be hop choice (beta acids and co-humulone and other things I barely understand). If you are looking for a more aggressive bitterness, try bittering with chinook or ctz. If your beer was a Simcoe/Amarillo/Citra IPA, it would probably be a lot more smooth and less noticeably bitter than the same IBU's with lower co-humulone hops. I also find noble hops produce a fairly sturdy bitterness when used excessively early in the boil.