Renstar you are right... 10miu tests are better off in the days leading up to when AF is due, and the 20miu ones are better after 16-18DPO... you need to have only 10 'units' of HCG in the urine for the 10miu to pick it up, whereas you need to have at least 20 'units' of HCG for the 20miu to pick it up.

I used first response midstream early test, which are great. 10miu, and can detect from 9-10DPO. Its possible that you are testing with a test that isn't sensitive enough.