I'm also experiencing intermittent lapses in AppData availability whereby successfully stored data is not returned when requested.
Please refer to this orphaned thread: http://developer.myspace.com/Community/forums/p/2913/14098.aspx#14098
The problem is sometimes profile specific, sometimes global, affecting all installations of our application.
About a month ago I implemented a workaround that re-attempts the request when the response is incomplete up to ten times.
It seems to be happening more frequently as time passes and now, when it does, the ten attempts rarely suffice.
This is understandably frustrating our users, so much so that I may need to store the association myself.