By 'suddenly', do you mean that earlier in this game you could craft with tenebrium, but now the same character(s) can not?
Have you tried using a recipe to craft, rather than 'experiment'? (ie something you know worked before)

What level of the Tenebrium ability do your characters have?

Is Brandon still around? Did you go through all the speech options with him? Had you already learned the ability from the book?