I don't think I'm pleasing him in bed, HELP!

My boyfriend has told me at least twice that I tend to "push him out of me" during sex and it keeps him from ejaculating, this is while we are in missionary position and doggy. Im not exactly sure what he means by this because I don't do it on purpose and I don't know what he's feeling. We do other positions too but I personally would like to do these positions and it be enjoyable for him. Any tips on how I can stop doing this so he can enjoy sex with me more?