When you have a circuit that used to work properly, and now some things are dim and some not working at all what you likely have is a circuit that has reduced power available. The brightest lights formerly are still working dimly, because their filaments are very thick and easy to push current through in a parallel circuit. The normally dim lights have very thin filaments, and now in the power reduction from ?? the current through them is very little, hence you don't see them at all.
Circuits have parallel loads, so to troubleshoot this you have to determine whether it is the positive feed having the problem, or the negative return line. Either line would cause the same problem.
