Most common issues that cause this are (from testflight's website):
- Device storage is full
- The provisioning profile is a developer provisioning profile
- The ad hoc distribution provisioning profile is corrupted and the device is having an issue with it.
- The device was restored from a backup and is causing a conflict for over-the-air distribution
- There was a network timeout
- Architecture settings of the build and the device are incompatible ( can sometimes happen -when "Build Active Architecture Only" is on when building).
- Not Using Mobile Safari.
for me it turned out that my client's ipad was running iOS 4.2.2 and my project supports 5.0+.