I am guessing your bike is the original 6V system? I am a fan of Lucas electrics, but they did have both good and bad ideas. One of the bad ideas was the 6-volt system charged by an alternator with no real regulation, it just switched between a low and high charging rate via the headlight switch and it charged lower at low rpms and higher at high rpms. It usually boiled the acid in the battery out all over your chrome and paint.
The only reason the bike would not charge is if there is a break in a circuit or a bad component. If you do a bit of research you will find instructions to check the alternator condition and output, the rectifier condition, and the condition of the wiring and it's connections. There is no magic-wand to fix a motorcycle, it is just a matter of educating yourself and putting in the time to check what needs to be checked.
I put thousands of miles on a Norton with a 6v lucas alternator system and it worked, but not as well as the later 12v systems that use a zener diode to keep the voltage closer to where it needs to be. Lucas used to sell an over-the-counter kit to convert from 6 to 12 volts, but with a bit of education it is a pretty easy conversion for anyone to do.
The 6v alternator can be made to put out 12v by tying two of it's output wires together, then you just need to add a zener diode on a heat-sink in the charging circuit to bleed high voltage to ground. I was able to rewire my 62' 6V Norton this way and keep all the original switches and hide the heat sink and zener so nobody knows it has been converted from the outside of the machine, they would have to look in the battery compartment or have me tell them were to look for the zener to see that it is now a 12V bike.
