Kind of random, but I just noticed that my credit count went up by 1 after making my first ever post in the General Discussion section. This gave me the theory that the lack of credit generation is specific to certain sub-forums (I suspect the newer ones don't generate any credits, e.g. Angelic Saga, Osawari Island).
Edit: and my credit count just went up again so yeah I think we can corroborate this theory.