What xconditions can (will) prevent a form class from being modal?

loform = create('aModalForm')
loform.show(1)
loform = ''

define class aModalForm as form
ShowWindow = 1
WindowType = 1

Function QueryUnload
  This.Visible = .f.
endfun

enddefine

This is not the same environment as that which is preventing Modality but this represents the override form settings which are employed.
At the time of instanciation the screen is not showing and there are no other forms open. There is a complex environment but I do not know what to look for.

I just need some ideas as to what can prevent modality.

Thanks

Glenn