I agree with your first comment. it could just be that you are not producing enough of the hormone for the pregnancy test to pick it up.
Well sometimes if you ovulate later than you thought it could cause a delayed period but then again your HCG levels could just not be high enough to detect on a HPT. You could try going to the dr to see if they will do blood work but maybe wait a few more days and test again. Maybe by then there will be enough HCG in your urine to detect if you are pregnant.