What helps gain a work permit?

What factors are taken into account when the game is deciding whether to give a player a work permit or not?

I can think of quite a few brazillian stars who've gone on to be great but would of joined me had the work permit been accepted.

Does things such as International Apps, Club Apps, Goals etc come into account when deciding work permits? And does nationality have an extreme effect on it. E.G Your an African player so it may be easier to get a work permit than a south american player?

Anyone know? Thanks.