Jon, I presume you are talking aout tenor banjo, The problem is that when most tunes are played they follow a patern and your fingers learn to use the familiar paths but sometimes the path changes and your fingers get confused. This is where you need to concentrate a little more to get back on the right path and "blaze a trail" for the next time. You will find this happens more and more as your playing progresses, not because your brain gets bogged down with new tunes, but because the tunes you choose to learn will be more involved and intricate. You will find that the easier tunes do not have to be learned but just play naturally, so you will always be learning new challenging tunes and therefore following new paterns and training your fingers to do the same.
Bill