Sometimes, the bobbin is not down completely on the post/spindle - then, the thread can wind unevenly, filling the bottom before the top. This results in the finger that disengages the winding mechanism not kicking in soon enough and thread winds beneath the bobbin itself - ask me how I know!
Even if the bobbin is down as far as it will go, you can get an unevenness to how it winds resulting in thread winding beneath the bobbin.
The fix is either to adjust the finger so it disengages the winding sooner, or guide the thread for the first little spell of winding so that it fills top, middle and bottom evenly (to start). Once the bobbin is even, it should continue to wind properly.
If you need more explanation, PM me. I have instructions on how to adjust the bobbin winder for my machine (P3 by Pfaff) that are likely similar.
That's if your dealer can't help of course, that would be my first call. (It was mine!)