Well, Nagai wasn't pleased with the English releases of Devilman in the USA. Devilman didn't do well in France. Only Grendizer is really popular there.
We usually get English releases of his anime, it's only his mangas that are a real problem to deal with. I heard an unconfirmed rumour is that DP wants to release the mangas in a particular order, so they won't just accept licenses of Devilman without starting with Harenchi Gakuen or something like that.