Hmm just remembered something. My virago 535 had problems last winter with regards to starting. Some days it would turn over fine and others it wouldn't even make a click. I managed to work it down to the regulator/rectifier. These are a common fault on the 400's & 250's. After replacing it (5 minute job on that bike, and even easier on bandits) she runs a treat
I thought it was battery problems, but the battery was fairly new (1/2 a year old at most) and when testing it all seemed fine.
If your engine isn't even beginning to turn on some start ups then it's definitely not the jets, so don't go playing with these. Even if you did need more fuel at idle you can just adjust the idle mixture screw, however seen as the bike is running fine at other times I would leave this alone.
The battery could be at fault however it would at least click when you try and turn it over, and the lights would dip.
Check all your earths especially the rectifier one. I'm pretty sure this is your problem.