I was told at my 12 week scan it’s all to do with your weight, if low body weight then it’s much clearer for people and visa versa x
Make sure you have a full bladder. That can help and they may ask you to empty it half way through if they need you to
I've tried googling this so much and it seems like it's just one of those things. My scans with my son were overly clear so not expecting these ones to be either