Tasks disappear over days

Hi all,

Using planner, every so often I find that a couple of my tasks
disappear over days.  The first thing that I normally try is to do

C-u -15 M-x plan

but this does not help.  I have to timewarp back to the last date
which held all my tasks, and then (day by day) step forward to the
current day.

Why does this happen, and what is the appropriate way to handle the
situation?

Regards
Stefan