If your slipper is coming loose, that's likely why it's not shifting, or sounding like it's shifting. When it shifts, the gearing is taller and requires more torque. With a loose slipper, that slips instead of turning the transmission.
The slipper has a hex-washer thing against the spur, then the spring, then the nut. If it has slipped quite a bit, then your slipper pad is likely glazed over and you will need to pull the spur and gently rough up the slipper pad. If the slipper pad is burned up, you will need a new one. If it's glazed, it will slip regardless what you do.
If the hex-washer thing is there and the spring as well, then your nut is likely worn out and you need a new one. You can try orange locktite on it. Might hold. Red loctite may be too aggressive, but blue likely won't be enough. I use orange on things I need a bit more hold than blue gives.