But it just doesn't go in.
Hopefully he isn't trying to press himself into you like near your clit.
Does it seem like he's coming too much from underneath, or too much from above? Some girls may have a big wide butt but it's flat right next to their vagina, and others may be much smaller but it could stick out more right there.
He needs to line himself up right at the opening and press through.
If you lift your torso away from the bed, that would probably help.
Then once he's inside if you lower your torso the angle will create the friction again that you had an issue getting himself inside of you.