Franco- Short, wide boards will turn upwind and stall quickly if you aren't careful with your sail handling. Perhaps this is what is happening to you.
One way to avoid that is make sure that you bring the sail up in front of you (almost so that you can look at the nose of the board through the window of the sail) before you grab the sail with your back hand and sheet in. That will prevent you from stalling and sideslipping. Also, make sure that you don't "oversheet" the sail, i.e. never pull the clue of the sail past the centerline of the board or upwind of the mast.
You might want to check out these guidelines for "how to teach windsurfing" and the links therein. It includes some information on how to avoid the problems you're having.
PS- Yes, short wide boards ARE slower than long narrow boards in non-planing winds, but with the right technique and a properly-rigged, appropriately-sized sail, they can do ok.