I'd like to start by apologizing for my english. It's been a while since I've stopped studying it and it's a bit hard to express everything I want to say
Well, today I made time and managed to fix this problem. Unfortunately I only had time to repair it, but not enough time for a DIY.
Short story:
I've taken the flap assembly out and opened it. I managed to do this without unplugging the wire from the car. In the assembly there is a connector for the electrical engine and if you pull hard enough it will come off without damaging something. I've done this by mistake. Unplugging the actual wire from the car is really time costly and a waste of time, in my opinion.
Anyway, I've opened the assembly and found out that the electrical motor works but might be to weak to do it's job. I've tried all kind of things to make the motor work better, but nothing worked after I've put the back of the assembly. It only worked without the back on. In this case I looked closer to the pin that comes out (the one that opens and closes) and I think that this might be the problem. At least in my case. Inside there is a spring that is pressured by the back panel of the assemble. That spring is too big/strong and presses to hard on the locking mechanism and won't be able to lock the pin, since it's not in a good position. The electrical motor doesn't have enough power. But, if the pin is in the right position, the locking system works every time. In this case I've cut 1-2 cm from the spring. It's not much since the length of the spring is about 10 cm. After cutting the spring, I reassembled the mechanism and now it works every time.
This was in my case. I think the spring is too hard and forces, in time, the pin to stay lower than it should. So, the electrical motor won't be able to lock it.