Thanks for the advise, but...

I switched some years ago from using Form Controls to using ActiveX Controls on worksheets due mainly to the Enabled function which I prefer for showing whether the control is valid for use or not [eg waiting for valid data entry before being enabled].  With Form Controls it was a case of either using Visible function or a Msgbox to deal with this.  Anyway ActiveX controls have worked fine up till now, and still works fine on other similarly configured sheets.  Hence, I would appreciate to know a bit more about why the problem has appeared.  It seems to me to be related to the Freeze Panes situation, with the controls losing focus in the window below the freeze row.
The spreadsheets are used for complex design involving sensitive IP so I regret I can't share/show them here.  Trouble is I've been unable to re-create the problem on any "demo test" spreadsheet - hence the frustrated call for expert help...!!!