Vent (?) windows don't close completely

Hi,
 
I have a 1967 Cadillac Deville Convertible and the small vent windows on each door don't want to close completely.  They open and close using the switch but they don't close completely.  If I push them in to close them, then the switch cannot open them anymore (and I have to push them out a little before the switch can do the rest).
 
Does anybody know what's wrong or how I can fix this?
 
Please let me know,
 
Thanks.
 
Jerry