I have a separate groups forum and have been told by a student that its 'not working properly'. I logged in as her, then saw that the forum gave the error 'Adding discussions to this forum requires group membership.' even though she is in the correct group and has added a discussion already.
I realised that I had changed the course default groups mode from visible groups to separate groups and that this had changed the forum too. When I set it back to visible groups, the New discussion button reappeared.