I just replaced the gaskets on 3 of my stationary windows. All were pulling away from the frame and when I took them off you could see why. They after time got somewhat brittle and less flexible. I order the gasket material from Hehr. I was having a difficult time trying to push the gasket into the frame and window. Then the light went on
After lubricating it with silicon I rolled it in at a right angle to the glass/frame, works great. Here are some pictures.