I started the game with GCD, then in the middle of the game I changed to Madd. But I immediatly changed to GCD (I dont even know if Madd was working well), and now my skills don't go up. When I reach 100/100, the GCD skill increase message appears but the skill level remains the same and the bar goes back to 0/100. How can I solve this?

Maybe you should try to restart the calculation processes of GCD. You should find all info inside the readme. Im not using GCD at the moment, so I can't be more precise. Don't know if that will be of any help though, but it's a try.