G.T.
Shortly after my son and his wife bought their first house they noticed mold on their windows too. They spent many days cleaning but never could get it to stop growing. The windows were brand new just 2 months before they bought the house so they were fine. They even had a rep from the company that made the windows come and inspect them to make sure. Then they had problem with their furnace. When the repairman came he asked them why they had the humidifier turned on? They never knew it was on. He asked them if they had problems with mold on the windows? He said they didn't need it on because there was enough moisture in the house and that too much moisture would cause mold to grow on the windows. Before he left (after fixing the furnace) he turned the humidifier off for them. They have never had problems with mold again! Even the mold that was on the windows (in the cracks) died off and they were able to clean it up.
Check the humidity in the house. You may have too much. If you do not have a humidifier running it could just be natural humidity. You could need to get a dehumidifier.