Hi max,

blastocyst transfers historically present with lower HCG levels. While most embryos implant 5-7 days after fertilisation, embryos transferred at blastocyst stage generally don't implant until 6-9 days after fertilsation. If your little morula was lagging a bit too, that would also likely mean it would implant later as it developed slower, therefore your HCG levels are 16dpo would of course be lower than most natural pgs.

I would definitely wait for your next BT, and I personally think it's as likely to have doubled as anything. Good luck.