That's the simpler type of coin acceptor, commonly called a "coin mech", not a comparitor. (a comparitor is electronic and it electrically compares a sample coin that is installed inside the comparitor to the coin a player inserts).
Your coin mech is labelled for nickel use, so I guess that is what you are using. Maybe I'm wrong but it doesn't look new? You may be able to examine it and find a way to force it to let all nickels fall thru as accepted coins, since the machine is going to be used in a home environment verifying the player's coins isn't all that important. The coin mech should have 2 coin paths inside, one for accepted and the other path for rejected coins. There is usually an adjustment for the size of the coin, if set wrong it will reject coins, you can try setting it to work better for nickels.
Also, I think these coin mechs have a magnet, that magnet may be interfering with the nickel. A nickel coin is mostly copper but has some nickel metal with is magnetic. Although copper is not magnetic it affects the magnetic field of a magnet in a way that will slow down a copper coin. You can try moving or removing the magnet to see if that helps. There may be a screw adjustment that moves the magnet further away from the coin path, sort of like a sensitivity adjustment. Examine the magnet and the metal plate it is mounted on, see if there is a screw that raises and lowers the magnet. If so you can try turning the screw and moving the magnet further away. If there is no adjustment you can try removing the magnet entirely. The magnet is there to catch metal slugs that people may try to use instead of a real coin.
Since the coin mech is mechanical you can test it and play with it while it is outside of the machine, to get an idea of how it works and doesn't work. I have attached 2 documents about your coin mech from the company that makes it. They explain how it works with nickels and how to make adjustments to it for proper operation.