I got a "brand new" battery once, had a short inside that read 12.6 volts on a meter but the second a load was placed on it, nothing. As Tools said, clean the battery connections, but also clean the connectors between the stator and regulator/rectifier. Then, start the bike, hook up a meter and rev the bike to at least 2000 rpm, better to be at 2500 rpm. Your voltage should be 13-14 volts. If it is not, then if you have to stock regulator, try to adjust it and see if that helps.
Also, when you ride, keep in mind these bikes charge just about nothing below 2000 rpm, just the way they were made. Many of them also put out very few amps for charging, so short rides will really drain things. You should clean ALL connections, or replace them with new hardware. Changing to led bulbs can also help reduce load which helps with charging.
Anyways, if the charging checks out, then as Tools said, it could be the starter being hot and turning slowly.
Does the bike start fine with the kicker?
The ride IS the adventure. The destination is just to get gas!