iPhone Ad Hoc Failure

Question

I am distributing an iPhone app to beta testers, some of whom (with iPhone 4 or 4s, iOS 5.0.1) have reported that the provisioning profile loads fine, they see the standard "installing" progress bar on the screen of the phone, and then, just before the installation is complete, they receive an error on the phone stating

"(Name of app) failed to install"

I'm using XCode 4.2.1. My phone is running 5.0.1 on an iPhone 4s myself, and I have no problems loading the app via Ad Hoc installation.

Any ideas as to what's going wrong? Does this indicate that there's some problem with the provisioning profile?

Thanks, James