I selected Diplomacy as favourite skill, but every points is still cost one skill point.

I even found a shield with +2CHA but diplomacy doesn't increase.

Is it a bug?
Should I restart the game?

You can cheese diplomacy i think?

There is a book you can find, i forget which one?

If you have two computers, start a new multiplayer game with a new idiot guy.

export your main guy then join that multiplayer game, play through till you find the book, drop it on the ground near a spawn point, save the game. Then your main guy use the book then export him.

Reload the idiot guy game with the book on the ground that you saved before you used it. Rejoin with your main guy, that you exported after using the book the first time.

Repeat as many times as you like.

Diplomacy taken care of!

You can do this trick for other stats but i recommend to avoid cheesing combat stats because that will spoil your game enjoyment in the long run.

Yes, you're right. My skill is above 80.

Still I don't understand why CHA work when I create the character but doesn't work when I increase it with items (it increase mana but not Diplomacy).

Do you have any other stat boosting item? Maybe it's because skills from stats are calculated only at tne character creation?