I understand now. Sorry for the confusion.
I have recently been distributing an app with a custom toolbar, and it works fine for my users. Differences are that my toolbar is global, not form or report related, and it calls code directly instead of using macros. But I wouldn't think that would make a difference.
Do you have access to your colleague's PC? If not, can you reproduce the problem on a PC you have access to? You're going to have to isolate the problem by trying out several things in the failing environment, such as verifying that the buttons' Action parameters are correct, and stepping through code if possible. BTW, does the app run in a full copy of Access or in the runtime environment on your colleagues' PCs?
If I had to guess, the strongest possibility that comes to mind is that your code is trapping errors, and hiding them. Some environmental condition on your colleague's PC may be causing a runtime error in your code, and your error handler is simply doing an Exit or Resume Next instead of displaying or recording the error. The easiest way to check this is on a failing PC, turning on Break On All Errors just before you click the toolbar button.
Rick Sprague