You're welcome. If you can get some video of yourself, that could help you (and us) see more of what's going on.I will admit I am not consciously engaging any muscles in trying to remain horizontal, mainly because I haven't been told to do so. On my next session I will bring this up and try to work on it, thanks for the advice!!
The other thing to think about is how your tank is oriented on your bc and body. If it isn't straight (along the spine) and centered on the spine, I've seen students struggle.
Try to hold some tension in your buttocks, and look straight ahead (not down) while swimming.