Problems with hiding a window during a thread

As usual I’ve got a window with a progressbar in my application to show if a longer activity is happening. At the end of the activity a notification is sent to hide the window. Now I’ve got a couple of customers where the window isn’t hidden. And I can’t reproduce this problem at all.

From the notification manager for the progress window:

[code] If Message = “showProgressWindow” then
'do something

ElseIf Message = “hideProgressWindow” then

dim currentThread as Thread = App.CurrentThread
if currentThread = nil then
  theThread = currentThread
  CallDelegateOnMainThreadMBS AddressOf HideProgress
end if

'other messages
end if
The hideProgress method:

[code]Private Sub HideProgress()

if theProgressWindow <> nil then
if theThread <> nil then theThread.resume
end if

End Sub[/code]

If I add logging then I can see that both methods are executed. But still the progress window isn’t hidden. Any ideas what could cause this or how I could force the window to hide?

Mac OS 10.10/11. Xojo 2014r3.