Games : What is the drop rate for plate items?

I am hoping to upgrade my charge blade soon but I need an S. Magala Plate. I have fought the Shagaru Magala a number of times now, breaking all the different parts off it where possible, but the Plate still hasn't dropped for me.

Does anybody know how rare the plates for monsters are? It is more likely to drop if I break a certain part of a monster?