Its very bad to have messagebox behind some windows app...i have this behavior in past.
to get messagebox as a top of all windows try this syntax

messagebox("what wanting....",0+32+4096)

read this MS page:https://msdn.microsoft.com/en-us/library/aa445082(v=vs.60).aspx
vbSystemModal 4096 System modal; all applications are suspended until the user responds to the message box.