ORIGINAL: RCKen
Did you pull the clear backing off of the Monokote? Probably did, but just have to check just in case.
I've done many Windex jobs on Monokote and have never had a problem where it did not adhere. If you had an excessively large piece of Monokote it may take it quite a long time (24+ hours) before it's finaly set.
Ken
Good answer Ken!! I have done that and ended up swearing at the covering before I discovered my problem.