Welcome to icliniq.com.
Firstly, is your period regular or irregular? Were you breastfeeding or taking any pills? In these cases, LMP is not considered as accurate, and scan is the main thing to determine the gestational age.
Secondly, the baby position does not play a role in hindering having measurements. The baby would sit on the lining because it gets nutrition from there, and that is what is called implantation.
Thirdly, what concerning is that if measurements were the same three weeks ago why the baby has not progressed in these three weeks. Possibly one of these scans is not correct either this recent one or the last one. This thing is worrying for me, and it is good that you are having a follow-up scan, and if the baby's heartbeat is there, then you have nothing to fear. Hopefully, everything would be clear by next week.
I am wishing you all the very best.