Sexual difficulties

how come in some couples, the guy will have difficulties inserting his penis into the girls vagina, as in the penis can't really penetrate into the vagina at all? why is this so? and there are any ways to improve it?

When a hard penis cannot enter in the vagina usuallay the reason is a certain unwanted contracion of the vagina muscles. Or abundant lubiracion or other medical help may solve the problem. (some alcohol for the girl may also help).

Maybe the girl is a virgin, or a recently deflowered one. Those are tight just because they've never been... um... used and hence need some stretching before the penis can go in without difficulties. Or maybe the girl is just shutting you out - when she contracts those muscles, it's pretty damn hard to get in.