Found it, I should of looked instead of asking.
In Preferences \ Sorting/Grouping in grouping there is an option to specify how files are grouped to an album. Mine was set up for Album Artist, Album Name, Size - so each song had to be the same size.
Clicking files for each album are in the same folder, restore the correct function.
I'm sure i have never set this up - ah well, not too worry it now works.
and again, Thanks for you time.