I asked a question a while back to which someone offered a very good answer to part of the question. Nothing has happened since. However, recently, I've made some progress on the remainder of the question. But I'm unsure of the best way to amend the question. I've got to introduce some new ideas, prove a couple theorems, and indicate how this still falls a bit short. My good sense tells me to summarize the new stuff in the main part of the question and add a new answer in which to include the details and prove the theorems, but I think that may be against the rules.
Assuming the answer to your old question was helpful, and the answer to it was accepted, you could just ask a new question with a reference to the old one and an explanation of what you've done since then. Asking a new question will probably be more effective at getting people to look at it.