There's a handful of reasons that can cause this. The probably definitive list can be found by combing the apple tech support discussion thread for it at https://discussions.apple.com/thread/2586385?start=105&tstart=0
I recommend you start at the end of the thread where you'll find the answers rather than at the beginning of the thread where'll you find the kind of posts that people write when they're tearing their hair out because their shiny expensive toy DOESN'T WORK.
The one that worked for me was:
- Restart the windows Bonjour service. If you don't know how to do that, just restart your computer.
- The other popular one is: "Sorry bud, you need a new router." Or if you're 'lucky' just a firmware update (Have you ever tried updating your router firmware? I did it once). You can't win then all.