I installed brand new 72-pin connectors into an NES given to by a friend. I also fixed the latch mechanism that holds the game down in place.
The first couple of tries, I got games to work perfectly after cleaning each one thoroughly with alcohol and an air duster to blow out any remnants of dust before popping it into the system.
After a couple days, I noticed some games just wouldn't work. I eventually resorted to blowing into the cartridge and alas, the cartridges worked instantly.
Does blowing into the carts do something OTHER than blow out dust? Does the moisture or heat in your breath help bridge the connectors? Why is this sometimes the only solution that works?