What Could Cause Negative Ovulation Test Results?

I have been trying to get pregnant for awhile now. Normally I am about 24 days. My last period was on 02/02/2011 - 02/05/2011. We started on Monday the 7th is it possible that I did concieve on this day? I started the ovulation tests on Tuesday 02/08/2011 but they have been coming back negative. Is it possible that I did concieve on Monday 02/07/2011 and this is why the ovulation tests are coming back negative now?