hey there
Well, my pregnancy has been trouble from the start. I'm now 12w6d (nearly in second trimester!) and i have still not been able to get a positive HPT. I went to the doctors when i was around 11 weeks pregnant because i knew something was wrong, i could feel it in my bones, even though the HTP had not confirmed my pregnancy. I had a blood test done, and eventually it came back positive. The doctor was in complete bewilderment, but she said it can happen at times, that the hCG is not secreted in the urine. I don't really understand, but all i know is i'm pregnant!
so yes, i guess this can happen. I have also read on the internet of stories that women have carried till full term with never getting a positive test, blood or urine.