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.
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.