I've had this problem on and off for years, with 2 MacBooks, that they took as much as a minute to fall asleep or failed to do so at all. At the moment my current theory is that Tuxera NTFS / NTFS-3G have been the culprits: it would sometimes use 30% CPU to do nothing and killing it let the machine sleep. And I think it's responsible for the problem I have that SpotLight doesn't work properly for several minutes after first starting up the machine and logging in - it seems to be indexing the BootCamp partition.
I've recently install the current version, 2012.3.4 and it was fine for a while – sleep and spotlight problem resolved. But they still came back and at the moment I'm working with NTFS not mounting the BootCamp partition.
Which is fine until I want to copy something onto it for use in Windows... Grrr. So at the moment my workaround is to use the Tuxera System Preferences Pane to manually switch it on and off when needed.
There are other solutions for NTFS support in OS X, but the Apple unreleased one isn't recommended which leaves you with the paid ones, Tuxera or Paragon, or the free Tuxera one, NTFS-3G.