moving to virtual desktops

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.