Ultimately the double glazing company you use is responsible for the quality of work and to ensure your windows are of an excellent standard. This is why it’s important to get a warranty and use a Fensa registered installer. If problems emerge within a couple of years then it’s likely the installation was of a poor standard.Contact the double glazing company and have the issue repaired or new windows replaced. This doesn’t matter if you received a discount of free windows as part of the deal. As long as you entered into a contract the company is obliged to restore the windows.