Because you are blanking out the critical combo properties.

I would accomplish this by having both a combo and a textbox in the fomr. The textbox is initially Visible = .F. the button click would be something like...

WITH Thisform
   .Lockscreen = .T.
   .Textbox.Value = Thisform.combo.displayvalue
   .Combo.Visible = .F.
   .Textbox.Visible = .T.
   .Lockscreen = .F.
ENDWITH

Now the user can edit in the textbox. When tyhey are done just reverse the sequence.