There is a part at the bottom of the window frame, which I will call "rail". It is shown on one of my photos. I am not sure if this is the proper name of this. The bottom of the frame for the pane straddles this, and it slides over this.
I'm noticing that the "rail" rises a bit towards the left end. I think this is causing the bottom front of the sliding window pane to tilt slightly up when it is pushed closed, which in turn is causing the window to stay ajar up at the top.
I'm wondering if I can remove this "rail" without destroying it. If Ican remove it, I am planning to raise the middle of it so that back end of the pane can be even with the front when it's closed.
The following link has two photos of the window: