If you call the Messagebox() function and it does not appear or only appears sometimes check the variables are not null. The message box will not be shown if any arguments are null.

In addition Jan Hink also pointed out that a messagebox does not appear in these cases.

1) messagebox function is called asynchronously (for example from timer-event) and another response window is open
2) messagebox function is called from modified-event ( not always )
3) messagebox function is called in open-script of other response-window (in PB4 and older)

In this cases, messagebox returns -1.