Whats going on? Help!!?

So I am 17dpo today. 2 days late as my maximum luteal phase is 15 days (ranges from 13 to 15) . I confirm ovulation with bbt and have been tracking for years it has never gone past 15 days unless i was pregnant. I have confirmed ovulation with bbt so I definitely know Im 17dpo. I tested last on 14dpo and it was completely negative. So I am confused as I know implantation can happen at 12dpo at the very latest and by 14dpo it shouldve been positive. I use very sensitive tests (10miu) . Could it be a chemical that has got my hcg levels to only under 10 so not detectable by tests !?