OK, interesting.... I think we've covered the "obvious" ideas now though.... :unsure:
(
apart from maybe checking things like;
the task isn't a milestone is it?
can you create ANOTHER task in the plan and add that OK? (if so then the original task maybe "corrupted" - can you just delete it?)
)
This thread
Cannot See Task In Timesheet has a lot of infoirmation, including some SQL that you can use to do some detailed "why" analysis.