Why doesn't it stay in? Sorry if TMI

Me and my SO are trying for #2 for 3 months now and nothing. I had the implant taken out 10 months ago and 3 months ago I found out I have PCOS. Well anyway every time after we have sex I have to lay down and keep my legs together otherwise all of his cum just comes straight back out again. 
Is his normal? I didn't have this problem before and I'm wondering if it's something to do with why I'm not getting pregnant along with th PCOS 
Thank you x