Secondary infertility - did they ever find a reason?

TW: living child

I’ve got a child conceived naturally nearly 4 years ago, very easily. Gave birth vaginally, no c section but instrumental delivery. I’ve been trying for no2 for nearly 2 years. Have some euploid embryos. I’m 30. Not sure which way to look for investigations, doctor is reluctant to do Emma or Alice, or even treat me with antibiotics if Fertilysis finds an infection (he recommends just probiotics…), says he doesn’t recommend doing them even after a few failed euploid transfers, because claims it’s a numbers game with transferring euploids, rarely a uterus / environment issue but Reddit makes me think it’s often environment.

So for my fellow secondary infertility gals , did they ever find a reason considering we’ve managed to successfully conceive and birth a baby at least once already?