I LOVE your new thumbnails, no. 8 looks so polished! My favourite. 10 and 11 look great to me as well, possibly 13 as well but I'm not sure whats going on there.
Why don't you buy Sammy Hall's 'Evoke the Dragon' downloadable content here? It is a fantastic video in which he shows his process of refining thumbnails and doing composition and colour studies before moving on to the final image. Maybe that's what you struggle with? I can't give any advice I'm afraid, but you might find that video useful. (Sorry if I sound like I'm trying to do some cheap advertising, just a thought^^)
Looking forward to your next steps.
//Edit: Sorry for some reason I can't connect to the ConceptArt store, here is the promotional video: