I have a Triumph Street Twin Motorcycle (2016) that has generally low milage and running well over the years. When trying to ride recently the bike stuttered and engine didn't kick in, after a few repeated attempts the stuttering didn't happen either and just no start.
Thinking the battery might be low (due to infrequent use) I used a jump starter battery and bingo! Working bike! However, after some riding and giving the battery a chance to recharge, once turned off the engine refuses to restart again.
Next I tried removing the battery and using a mains charger to full recharge the battery, after some hours and the charger indicating the battery was at 100% I popped the battery back in and still no luck.
So a fully charged battery won't start the engine, but an additional jump starter battery will.(Connected to the same terminals / contact points)
What could the issue be? and how can I fix this at home? (Given this has taken a while so far, my MOT has lapsed so getting the bike to a mechanic would be technically illegal)