As has been suggested, it is probably your pads sticking to the front discs. I continually had this problem with my 4/4. I've resorted at times reversing out of the garage with locked front wheels until the pads released with a bang!
I got to the stage where I would go into the garage every day and rock the car back and forth to stop them sticking.
Changing the discs and pads on the Roadster means I don't have the problem any more, the discs don't corrode, but it's an expensive solution.
