Why isn’t doggy style working?!

My new boyfriend and I can’t get doggy style to work, which is unfortunate because it was one of my absolute favorite positions with my ex. My boyfriend is inexperienced (I’m the first girl he’s been with), and we just can’t get it in in the doggy style position. It wasn’t a problem with the past guy, he would just easily put it in while I was on my knees. So I’m wondering if anyone knows what we could be doing wrong and how to fix it? I’m also fairly inexperienced so I’m confused why this isn’t working anymore.

*if anyone is going to complain saying it sounds like I’m too young, my boyfriend and I are 19.