Thanks for your help. We have managed to reproduce the problem, and my hunch was right. If the an item is created in TFS, it will not be given a Backlog Priority number in TeamPulse. You can avoid the problem by creating all new items in TeamPulse, or by re-running that script I sent after pulling in new items from TFS. The script will only update items that have no value in their Backlog Priority, so it is safe.
If these work arounds will not do for now, there are a couple other things you could do in your TFS mapping file to either set a default value for Backlog Priority or to sync it up with an existing TFS value. Let me know if you would rather do this.
I am happy to report, we are working on a code fix for this issue right now.
All the best,
the Telerik team