It's the same, alas, with writing a book. Changing books in midstream is not unheard of. Sometimes the book just doesn't take off. Like a tired polo pony, it loses it's ability to sparkle. Then, the only thing to do it change books and hope the sparkle returns. This pony rested and was played again in the last chukker - so sometimes putting a 'WIP' away until it gets fresh again is the best thing to do.
No use flogging a dead horse. :-)