Working for an independent press as I do, I often find myself looking at manuscripts that have been rejected by the larger corporate publishers. I'm not offended by this, as the big houses aren't known for taking chances, and indeed being rejected by them is not a badge of disgrace. We all know that this is an incredibly subjective industry.
What I have noticed, however, is that I do seem to get a number of manuscripts that have clearly been around quite a while. It's hard to explain why, but a manuscript written more than two years ago always has a bit of a stale smell about it. Often there are references to events that happened a while ago, almost as if they were contemporary. But more than that, there's a difficult-to-describe feeling that the manuscript was simply taken from a shelf, dusted off, and plopped in an envelope.
Here's what I'm thinking. If you have been trying to get interest in your manuscript for more than, say, 2 years -- and have had no takers -- you are most likely better off starting something new. Look at your rejections to decide whether you should give your character another try within a different context or plot, or whether you should try something new altogether. Remember, editors are always looking for something fresh, interesting, different. Something you wrote three years ago isn't likely to fulfill any of those criteria.