Three things: practice, practice and practice. OK, wait. Besides those three, ask around at the range. I've received very good advice from different people over the years at the range. Someone watching you might notice something you should change, e.g. your stance, your grip, etc. Hard for us here on the forum to advise much, but look around for someone who seems to know what s/he is doing and ask for advice.
A quick story. About 14 years ago I got tired of missing flying birds with my shotgun. I determined to figure out what I was doing wrong. I checked out a couple of books from the library that really helped me with my stance and mounting, and gave me advice about practicing mounting in my living room (with an unloaded shotgun). But I still couldn't hit much.
Then two things happened.
First, after all these years I discovered I was cross-eye dominant. Wow, knowing that has helped. But the second was maybe even more important.
I went to a range near my father-in-laws. There were two guys shooting skeet. I stepped up and asked if I could watch and ask questions. They were so helpful, in fact, they insisted that I not wait but to join them in that round. Those guys bent over backwards to help me. When I finished, I was on my way. Oh, I still missed a lot that round, but I hit several doubles, which I couldn't believe. I'm still no shotgun competitor, but I can finally hit running bunnies and knock birds out of the air.
So, ask around at the range. Oh, and don't forget the three "P's"!