It could be due to various factors like stress, ovulation timing, or minor hormonal imbalances. It is best to consult a fertility specialist or your gynac for a proper checkup.
I know how heartbreaking those negative tests are, I used to overthink every symptom. Please don't lose hope, sometimes it just takes a little more time.
Listen, conceiving can take up to a year even for perfectly healthy couples. Stop stressing, track your ovulation properly, and see a doctor if it's been more than a year.