"Slide in" can mean different things.
Like I said - some penises are too long to completely fit inside because they might hit the cervix. Also men with a huge girth can have trouble penetrating the woman when her body doesn't produce enough lubricant to ease it in before the vagina had the change to stretch. It's not a rubber band after all, it may need some time and a slow approach.
Just like some men suffer from phimosis some women aren't as flexible as others but it usually just takes a little more time and effort than usual.