Dear Sarah,
I recall (not using levers actively for a while) adjusting screws on the levers (not those attaching it to the harp but those holding different layers of metal in the moving parts of one lever). Please see if anything is funny in those screws or impede the movement of the different layers of metal of the lever. They are screws on on BOTH sides of the lever (your left and your right when you are looking perpendicularly at the neck of the harp). Before disassembly or even tightening/loosening, looking for missing screws or broken/bent metal first. Compare it with adjacent levers (check both sides of the affected lever.)
Please make sure 200% that you have the correct keys/screw drivers before you adjust the screws. Best to put the key in and softly emulate turns it to sense if there is any play or looseness before applying any real force. Also check that the key slots in securely and fully and is likely to take the torque of the turn. I would lower the tension of the string before starting.
The higher pitch side is easier, one slot screw and it is larger. The other side is a pair of small screws. You will have to flip the affected lever and adjacent one(s) to make room for this activity. Try different combinations to make it work. If you key is has a gigantic handle, buy another one.
In case I am not clear, you are NOT removing the lever from the harp. You are only tinkering with the different layers of the moving parts of the lever. I would try not to take out the lever or else you will essentially have to regulate that note of your harp yourself.
Good luck.