These are the following you can do to make it work, in my past.

1. Clean the NES teeth slot with a damp warm tissue, or something with moister. Avoiding using cleaners, asides official cleaners.
Avoid using alcohol.

2. Clean the game teeth. Same as above.

3. Do not blow cold air into the cart, You just helping dirt stuck on the game teetn stay there.

4. If that does not work open the console. And check the teeth. Two things can be wrong.

A. The teeth are not contacting the cart. Like the Sega Saturn, these needs to be pushed it, it will create friction but they wil contact corretly.

B. For my NES, I just push the pins because the pins might slip out. I am talking from behind, When you insert an NES in the original NES, the PINS actually move, out of place. Doing it repeatively will help this problem occur. Basically like me you just have to push them in and make sure none are out.

5. Also if your NES is exposed to something, then that might be a problem. Water damage, toxins, electrical interferance, or something else.

6. The last thing it could be is the idea of bit-rot. Bit rot simple means after not being used for a long time the data is
unrefreshed. However this is a   1% out of 99.999.9% chance of occuring. Is less likely to even occur. Like I have a brand new game, and it has a fully charged battery but it does not work, because of bit rot.

7. if your using pirated games, clones, then some do not work, period.