We prefer you to raise new posts in general.
You can revive old threads when you are making nice comments about someone else's photos of orchids. I do that quite a bit when I find ones I've missed.
But if you are asking a question it's best to start a new thread because often you are less likely to get answers on an old thread.
We don't mind the same questions being asked multiple times because everyone's conditions vary.
I actually mention some of that in my 'Guidlines' post
here.
Really our preference is all about members getting the attention they need for their questions, pictures or whatever they are posting. We don't mind old posts being revived, it's just experience has shown that people get very few responses when they do that.