Dealing with window contractor re: warranty issue - need input

[sbj405]

I could use your input on a situation I have been dealing with. I had windows installed 1.5 years ago. I checked the company out and they appeared reputable. I have a 5 year installation and 15 year warranty on the windows. Well one of the windows is very difficult to open and screech. I see that something is rubbing. It's probably not that big of a deal and I'm sure they could fix it quickly. I don't want to mess with it and void the warranty or anything. I have attempted to call the contractor over the past 3 weeks. I have left probably a half dozen messages for the service mechanic. Last week I spoke with the owner who assured me the service mechanic would get in touch with me to come out and look at it. Well he never called. I have left 2 more messages for him (who I'm now told never comes to the office) and the owner and now no one will call me back. Am I just being impatient or is this unreasonable? What should my next step be? Written letter?