well it's hard to give advice...I think you know what you are doing. I myself am wary of the concept 'to develop EVERYTHING from one single idea'. Maybe because considerable technique (whether learned or not) is needed to cause a single idea to continue sounding interesting. I mean, you can invert it, change this or that and preserve the idea, or reharmonize it, yet it can still sound as if 'it's not enough'.
I actually am kind of working on the same problem as you. I don't have it all the times, but it's probably the biggest challenge in the field of composition. I also don't understand why some books etc insist in preferably developing everything from a single idea. This concept, I think, stems from the fugue. But I have never listened to a fugue in it's entirety, funnily enough.
I think it's best to try several things and to stick with whatever seems to work best. Who cares about what Mr Professor says?