Thank you ROGER, let me give you more information:
The fin i use is the stock one that comes with the board, and it's a little rough on the edges because of some small scratches. I haven't done anything to it (didn't know it was necesary).
The wind range where i have problems is around 20-25 knots, with either 6.0 or 5.3 sails. i guess the chops are 30 - 40 cms, maybe a bit more.
With the tips on the technique i will try to improve and coment back.