The mega is powered through a buck converter that gives me 5V from whatever input i have (i wire the input directly to the fans so that i can choose the fan speed by adjust the voltage, while the buck converter makes whatever is at the input into 5V for my arduino, servos and so on)
However i have found the mistake. First of let me say that i am sorry that i have wasted everyones time, however it did give me some new knowledge.
The problem was that i did not define the pin as output, so it only gave 2.1V as high, and so the mosfet did not fully turn on.
I have written my code a long time ago and so i assumed i k\made it right, goes to show how you need to check everything from the most basic things before jumping to conclusions.
Thanks again.