IVF, PGS and miscarriage?

We have a healthy 3.5 year old that we conceived after a few months of trying with our first IUI and for the past 2 years we’ve been trying for a second. After multiple rounds of IUIs we progressed to IVF with a diagnosis of suboptimal fertility rather than unexplained. My husband and I are both 37 and I’ve never had a miscarriage but to maximize our chances of success first time round we chose to do PGS. On the antagonist protocol after 11 days of stims, we were able to retrieve 18 eggs, 15 of which were mature, using ICSI 11 fertilized, 8 made it to day 5. All were 4BB and above and after PGS it was confirmed that our 6AA and 5AA were the two that came back normal. At the time of the frozen transfer the first one (the 5AA) didn’t survive the thaw so we progressed to transferring the 6AA. Lining was perfect, progesterone was great, estradiol was great so were felt positive. 6 days post transfer I did a hpt and saw a faint positive which was great! We’d waited two years for that! I definitely let pregnant so things were looking good. 9dpt I had my beta which showed my hcg was 14.5 which I knew was not good at all considering I’d gotten a positive 3 days earlier. Two days later my HCG was 3 and now I’m just waiting for my period to start as we know for sure it’s over. So here is my question, can anyone suggest things for us to look into as a possible source of the issue? My tubes are clear, HSG, sonogram was normal.....obviously there’s something wrong but I have no idea what it might be.