I think I've found the thing that's causing the problem for me. It's a setting. In Display Settings, all three monitors are set to use the same Resolution ( 1920x1080). The 'Change the size of text, apps and other items' setting was at 125% for two of the monitors. For the third, the setting was 150%. When I moved the .xls between the screens, it was the third monitor where the problem presented. I changed the setting to 125% for the third monitor and now the buttons are working on that screen, as well as the other two i.e. it works consistently across all monitors.
Thanks again for everyone's attention and support. Nice to get it resolved !
Steve