This has been plaguing us for a while. For reasons unknown, stories and/or bugs refuse to show in Urban Turtle's Sprint Backlog. We have confirmed they are all in the same Iteration Path, all in the same Area, the stories and bugs are committed, etc. They show up fine in the TFS 2012 backlog.
Today I discovered that one of the missing bugs had a "backlog priority" of 41266102. I'm assuming this was automatically assigned by Urban Turtle while dragging and dropping, but I can't know for sure. When I change this number to 10 and refresh the bug now appears on the sprint backlog. My guess is that the auto assigned priority may be too large to be handled and is silently erroring.
I can't be sure this is the only cause of our problems, but it would be nice if this could be fixed.