Unlock and open the sliding window. Move it so it is all the way open. Roll out the retractable screen so it is covering the open space and can easily be accessed without the window in the way.
Inspect the screen from both the inside and outside of the house to look for any damaged clips. Locate the spring mechanism at the bottom of the window. This pieces slides along the sill and keeps the window from moving when it is supposed to be stationary. Verify that it is in good shape.
Replace any window clips that are damaged. Pop them off with a flat head screwdriver and install new clips. Pop out the spring mechanism if it is worn and needs to be replaced. Pry it off with a flat head screwdriver and insert a new one in its place.
Slide the screen back and forth to see if it is working better. If not, remove the entire screen for further repair. Push down slightly on the screen and pull back from the inside of the house to remove it from the track. Inspect the screen to look for damage on the frame.
Wipe off any dirt and debris that may be causing it to slide improperly with a clean rag both on the screen and in the track. Reinstall it back into place. Set the bottom section in and push down so the top can fit into the track. Test it out again and if it is still not working properly remove it just like in the previous step and install a new screen in its place.