I've found at times that starting with isopropyl alcohol doesn't get enough of the oxidation off for it to keep working normally more than a day or two, while the fiberglass pen being used once feels like a more permanent fix. (Maybe after 10 years it will have issues again, but it at least for sure keeps working for weeks or months). I had an issue of this sort really bad with a GBA power switch. You can try to get a little alcohol in from the outside and move it back and forth and maybe it seems to work for 1-3 days, but then it would be intermittent again. The proper fix was to desolder the shielding on the power switch to get inside, and then clean off the oxidation from the contacts inside.

I should've also mentioned this in my first comment, but I recommend opening up the cartridges before cleaning them. I don't think you can get at the whole contact properly while the shell is on, and it's easier to do an even up and down motion along the each contact when the shell is out of the way. I go back and forth for a couple seconds on each one until it looks shiny. As a bonus you can possibly find out if your game is genuine or counterfeit while it's open and the chips are visible.