OK, it must have been a corruption somewhere in the .jap file
I increased the text size and moved it to the centre and then rebuilt the album using the Plain skin, then with the Responsive skin and back to the original Tiger skin. I could then make changed again to the text size and position and the images are now OK.
However the other skins have changed some of my other Tiger skin settings so I need to recover those.
Have you saved your settings as either 'Default' or 'Tiger default" ? See
https://jalbum.net/help/en/JAlbum/Menus#Save_as_default
Or saved a backup of your settings, see
https://jalbum.net/help/en/jAlbum/Preferences#Save_backups_of_projects_.28settings.29
Links are for current version of jAlbum