I have had a similar problem on occassion. The first thing to try is a trick from John Palmer's book: shake up your bottles to resuspend the yeast. This often leads to another burst of fermentation/conditioning in the bottles.
This trick has also saved a couple batches of 'ruined' beers for me when I had let primary fermentation get too warm (> 24 degrees C), and off flavours developed. I let the bottles condition for a month, then shook them up to resuspend the yeast, and presto - a month later they tasted completely improved.
It doesn't always work, but it is a technique I swear by.