No, they aren't part of an option group. Only other thing I can think of is that the database was converted from Access/Office 97 to 2010. The conversion caused a couple of issues but they are resolved and AFAIK are unrelated.
Addendum:
Tried creating a new database (in 2010) with ControlTip text in a variety of controls. They all worked as expected so it looks like my project database may have become cocked up in the conversion from 97. I suppose I could delete and recreate the form but it has over 100 controls so I don't really want to do that. Maybe I'll try deleting/re-installing the offending controls, or something else if anyone can suggest it.
Addendum 2:
OK, deleting/re-installing worked. Also realized all the controls that don't work are the ones that came with the conversion (the working controls were added later in 2010). Same thing BTW for text boxes. :banghead: