Window problems
Hello,
The vinyl master frame should have a spring adjustment on each side near the center of the window channel to bring in the frame and prevent what is happening to your window. Normally, you would turn the adjusting screw clockwise to bring in the center of the master frame but since so much time has passed, the caulking has dried out and prevents you from doing this.
You would need to cut away the caulking outside and inside along the window master frame about 12" up and down from the center, slide a flat bar or chisel to break the seal, and bring the center in. You might also have to use a shim if the adjusting screw won't bring in the center of the frame. When you're done, try the sash to see if it is operating correctly, then re-caulk the frame.