Lining on transfer day?

[Hopethisyear]

I just did a mock cycle with my clinic and instead of doing the transfer after 5 days progesterone we did a biopsy instead. I've had lining issues in the past and not able to get to 7. I asked my Dr since it was a mock cycle if he would check my lining on biopsy day (mock transfer day) and he said no as it wouldn't be accurate. I wanted to know what my lining would have been at transfer. Any insight on why he wouldnt?