HPT BFN when you're actually pregnant: How often do you think it happens?

[Moondance]

I'm currently ttc.... 12 days late and getting BFN's.
I have heard of a few other people getting early BFN's when they are actually pregnant.... But I often wonder, how often does it happen?
All the boxes like to say they're 99% accurate, but then all these people I hear of getting BFN's. One lady I know, right up into her 6th month, she was still getting BFN's on HPT's despite knowing she was pregnant because a blood test had told her so. She just wanted to know how long it would take the test to register.

So is it more of a rarity and some people are odd, or it happens more often than the manufacturers want people to think?
What do y'all think?


ETA: My doctors clinic is the only bulk billing clinic in town and is very overbooked. Telling me to go to the doctors... not helpful. Thats not what this thread is about. I'm curious on peoples opinions regarding whether or not negative HPT's when you're actually pregnant, are possible. I ask because yes, I am experiencing negative tests, and I'm aware I need to see a doctor, but thats not what I'm here asking for.