Contextual Arabic kerning doesn't work

I have an Arabic font that I want to use contextual kerning (it has quite a number of cursive attachment, so contextual kerning is necessary). However, even the simplest two-glyph context does not work. Actually, is contextual kerning working for any script? If yes, what do I need to do differently for Arabic?

Normally you just need to set the right lookup flags. I recently changed the handling of custom kern features so you might have a look at the feature code in the temp folder.

Actually I looked at the feature file, and if I understand correctly, the lookup is created but not being called. Even in Latin I could not make it work.