chemical in body that blocks getting positive??

So, I'm curious. is there by any chance some type of chemicals or hormone that could keep you from getting a positive test result? with my first pregnancy I didn't even get the faintest positive until 8-10 weeks now with this pregnancy I've been getting faint positives since maybe the the start of my 5th week (I'll be 6 weeks Monday) but the positives are so faint that it would be easy to miss the second line. what do you guys think?