moving to virtual desktops

[Zebee Johnstone]

I have 5 virtual desktops and sometimes they all have things on them
and sometimes not.

I have defined move one desktop right and move one desktop left to
cntrl-alt-rightarrow and cntrl-alt-leftarropw respectively.

This works fine, in that I move from desktop to desktop, whether or
not there's anything in them.

until suddenly it doesn't work anymore - I can only move between the
first 3 that are populated.

If I click on desktop 4 in the pager I can go there, but
cntrl-alt-arrow only moves me between 4 and 5, not back to 3 or 2 or
1.

If I run a program in desktop 4 that doesn't change the behaviour, I
can only go from 4 to 5 and back by keystroke, or between 3,2,1 by
keystroke.

If I right click on the pager, go to pager settings, change the number
of desktops, then right click again, go to pager, change back to 5,
all is OK and things work as I expect.

I haven't been able to work out what triggers this behaviour, doesn't
seem to be the starting of any program as such.