hahahaha I made this game many years ago, and I still enjoy getting comments on it! ^^
Maybe it was badly explained on the game (I don't remember now) but second and third sensor don't make it learn faster, otherwise it makes they learn slower, but able to learn more and better (in machine learning terms, they have bigger layers).
Sometimes learning process gets stuck on a "local minimum" and it is very hard to get out of that. The only advise I can give you is to made they to "unlearn" a lot to get away of that "local minimum", and then start again the learning process. To do that, get back to old maps for some generations, or change the configurations by A LOT.
The green bike is there to allow you to see if each generation is evolving or they are stuck on a local minimum and getting away from it: if the green bike is always doing the same, they are not learning, neither "unlearning".
Another hard option you have is to retire early, you won't get the bonuses from completing all races, but you will start again with the same amount of sensor to try again learning from 0...