This sounds like my first spring with Dita, she was a real pain for this.
Here's what we did, took the carbs off, and cleaned them. There was -sand- in the bottom of the bowls!!
So,.. it may have been a clogged starting jet that was doing it, but before that, I would crank and crank and crank, then when the battery was dead, I'd put her back on the charger and go to work. I'd come home and repeat the process.
At the same time we made sure that the spark was good, and the plugs not fouled (from all of the attempted starting). Those plug wires can be a real pain too.
When I start the bike, I usually try to get her running without the choke, then turn it on as she starts to run, of course, your milage may vary depending on your locale.