electrophoresis problem - (Feb/12/2009 )

Hi every one
I hope some people could help me. I'm quite disappointing and perplexed, because after performing a protein extraction from bacteria, with a typical 2D buffer (Tris, Urea, Thiourea, CHAPS), I quatify the extraction and obtain about 4 ug/ul, quite a big concentration. But in the moment that I perform a 1D-SDS PAGE, I don't see any band on the gel. Of course I mix the desired volume of my protein extraction with the Sample Buffer for monodimensional gels, in the correct proprtion (final concentration of Sample Buffer = 1X)
Do any of you have an idea of where is the problem?
Thank you in advance

If you really got 4ug/ul, then you should have seen the bands on the gel , how did you quantify your protein? May be the estimation is not proper coz urea can cause increase in OD.