I know I should "let them have fun" or something, but the game isn't really working.
I'm GMing a pathfinder campaign, but this isn't system-dependent. My players aren't roleplaying at all. Ok, they don't metagame (also because I try to keep all the information that the player shouldn't know for me), but they just "play" the game. We sit around the table for 3 hours rolling dice but no one is actually roleplaying. It's fun, but it would be a lot better if they roleplayed correctly. How, as a GM, help them to improve their RPing skills without ruining their fun?
Even if I haven't directly told them the problem yet (I thought It could be unpolite or something) I've tried to add a lot of details to my games, a lot of interesting NPCs, a lot of rp encounters and I always describe the enemies attacks during combat (so I don't just roll, but they won't do it as well). But they always do the minimum roleplay possible. They talk to the NPCs without roleplaying at all.
Best Answer
So before I start, and before I give any other tip, the number one rule is that if it's not broken, don't fix it. If everyone is having fun, maybe it should be better to leave it this way for now and to come back for it when it's relevant to all (or even most) of the group.
With that said, let's delve into the actual tips that I'm gonna suggest, hoping that at least one of them will help you:
Lead from example
As a GM it is your responsibility to make sure that the players know what they should and shouldn't do in the game. One of the best ways to show them what you are expecting from them is by giving them an example. Put many NPCs in the story and portray and play each one of them to the best of your ability. Male or female, dwarves or elves, you should showcase to them that there are persons behind those stacks of mechanical characteristics.
More than that, though, giving them the example will also show them (hopefully) 2 very important things: Firstly what portraying a character really is. The task often seems intimidating for new players and by showing them what it looks like it becomes more familiar, thus less scary.
Secondly it will show them that there's no reason to be shy, that looking silly or stupid is not a bad thing and that they shouldn't fear any of that. If you're not a pro actor and you're willing to do it, why shouldn't they?
State connections between the characters
Connections are everything. They are the fodder from which those shining moments of roleplaying come. Take a break from the game and spend some times with the players establishing connections between their characters to the others and between their characters to the world. It doesn't have to be complicated; it just has to give a basis for a roleplaying scene. If a character hates another one, for example, this feeling will come into play sooner or later. This leads me to the next point:
Rewarding great roleplaying
It's not enough to just give some examples and then move on. We have to reward them for those moments of roleplaying. At the start, aim for the bigger rewards, bennies or experience points or the like. They'll want those badly and because of that they'll go the extra mile to get those. Then, slowly but surely, they'll learn to see the benefits of roleplaying their characters and they will start to enjoy this acting thing. At this point you start to minimize the rewards (a +1 for an action, for example), making the story and the enjoyment themselves the real benefits. If done right, this will make them act for the acting and not for a bribery of any kind.
Make a Fiasco one-shot
I really think that this game should be a mandatory game to know. It teaches you so many things about how to play a character, how to connect the character with others and when the story should triumph the characters. Let you players taste it, show them what is expected from them.
Another benefit is that this game is both short and GM-less. The length lets you continue to the regular session a short while afterwards, thus implementing the lessons immediately. The GM-less thing helps them to see that not only GMs are supposed to roleplay, but the players too.