My preflight app is working on a some devices, and not others. They are all using the same provisioning files.
I rebuilt it again, and still the same issues.
What should I check next?
Check to make sure that the Device IDs (UDIDs) are specified in the Apple iOS Developer portal and included in your mobile provision files. Also make sure that your UDIDs were obtained through iTunes or Xcode and not through a third-party utility. Some third-party utilities display the incorrect UDID.
See Unable to install developer .ipa file for information on how to resolve issues with apps not installing. Note that unless you are using custom fonts you don't need to build your own preflight app anymore. You can install the DPS Preflight app from iTunes and use that.