old interior door knob can't turn

WD-40 is not a lubricant which is why it's only lasting a short time.

Use something to clean it out, carb and choke cleaner is reasonable and follow that with some compressed air.

Then apply some graphite-based lock lube to the moving parts. These are also often called "dry" lube and they don't hold dust like oil-based lubes can.

Once it all moves freely with the new lube, you should be good to go.