I think I may have found the answer. On the 4th level of dashboard there was a date filter which seemed to be corrupting the initial dashboard. I'll do some further investigation in the morning and post some screen shots if it's still not resolved.
OK, my thoughts about the 4th level filter didn't work out
However, I have now found the problem (completely by chance)........the back button was linked to a different data source and once I assigned this to the same data source, everything worked as expected
What I don't understand is why it worked for some selections and not for others.....one for another day I think