I'd bet the number one reason store owners don't like about their current window cleaner is the inconsistency of them showing up.
Be consistent on trying to gain their business. Pop in on a regular basis and hand your card to the manager or whoever is up front. Don't take up any of their time. Just say a quick hello and hand them your card with your price on the back. Eventually they will start to remember you and hopefully they'll give in.
That's what I used to do. And yeah, the chain reaction will happen. Once you get one store the others will start to trickle in, as long as you're consistant.