i have my original NES and dozens of games from my childhood, and i have found that it is difficult to get them to run. thanks for the advice, i will give this a try!
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.
Took me a bit longer than you but I was being super cautious having never opened a console before.
Highly recommend for anyone else with a broken NES lying around. Good chance the 72pin connector is at fault.