TC - sporadic backup on wireless

My TimeCapsule is only a week old, and for the first few days has worked fine, but a week in and its doing odd things.

My MBP is running 10.7.5, and Airport utility 6.1.

I have set the TimeCapsule to be in Bridge mode, as its connected to the gigabit ethernet at my office. Its password protected and only the laptop has access to the wireless.

Internet connection seems good - its very fast and seemingly reliable, yet TimeMachine has started reporting it cannot find the Backup Disc and to check the IP addresses etc.

Airport Utility shows green lights for both the Internet and the Time Capsule. The Backup disc appears in the list of discs for performing a backup, yet the wireless connection does not happen.

If i attach an ethernet cable, then the backup happens almost instantly.

Surely the point of an automatic wireless backup is to avoid manually attaching cables?

Any ideas what might be happening?

thanks

Adam