There could be a variety of reasons for that. I had a conversation with a wonderful lady who directed me to my future midwife. She was telling me the stories of her pregnancies and she never got positives on HTPs or the blood test with all 3. She also said for some women, their hormones don't reflect on tests but it doesn't mean you can't have a healthy pregnancy.
Wishing you a happy and healthy 9 months!!! :)