I recently fixed a neighbor's slider that was very difficult to open. They purchased the airplane and did not build it. The nylon block at the aft end of the canopy was too tall and rubbed on the fuselage skin beneath the track that is screwed to the fuselage. Removed the nylon block and put the bottom side of the nylon block to the bench mounted belt sander. Removing 1/16" was just enough to allow the canopy to slide easy.
On my almost 22-year old RV-6 that has 3,400 on the hobbs meter with slider, I take a paper towel, spray a little WD-40 on it, then wipe out the inside of the roller tracks. I also use the same paper towel to wipe the rear slider T rail / track that is mounted to the fuselage. Typically this makes my slider very easy to open and will sometimes slide aft with just the force of gravity on my tail wheel aircraft.