Mine does the same thing.
The only time it does it is on the first start when it's sat for over a week or so. I'm not sure what causes it so like a lab rat I have simply adjusted my routine. I roll it out and let the starter spin the engine over a couple of times. If it doesn't start, I just shut off the key, go take a couple of minutes to put my gear on, then it just starts right up when I try it again.
I don't know what causes it. It doesn't matter if I use the choke or not.
At one point I thought maybe there was some kind of binding in the choke mechanism: i.e. I would pull the lever, but perhaps something was hanging up so it took a couple of minutes before everything at the other end of the cable moved into position.
But, one would think this would also occur if the bike had sat for only 2 days and a choke start was required. If it just sits for a couple of days, it starts up just fine. It's just as cold when it's sat for 2 days as 45, so ambient temperature doesn't seem to be a factor.
Under the same temperature/length of sitting conditions, I've tried starting it with no choke, half choke, full choke. It does the same thing every time. With my new routine, I fully engage the choke and if it doesn't start, I'll just leave the choke lever where it is until I come back out and start it. I don't attempt a start, move the choke back to "Off" while it sits, then come back out and return the choke to "On" before attempting to start it again.
I queried this on another thread recently and a few owners had experienced the same issue, but it doesn't seem to be all that common.
I've been riding it pretty regularly, but the next time it sits for at least a week, I'm going to try Savage's suggestion of twisting the throttle a couple of times before attempting that first start.
It would seem the only thing that really changes between my first unsuccessful start attempt and my second successful attempt is application of vacuum to the petcock.
Another thing I've noted is that it seems to do it more toward the end of my riding season. Over the Winter, I dump a prodigious amount of Seafoam into my tank and make sure it circulates through the fuel system. Maybe after a few months of running shitty 10% ethanol gas, the thing just gets a little persnickety. I don't remember it doing this back in say, March or April.
I will have to make it a point to see how it acts again next Spring after it's had a healthy dose of Seafoam run through the carb again.
As a short-term experiment, I'm going to start filling up with the non-ethanol 90-octane gas they're now offering at a new gas station close to where I work. I'm going to keep my tank filled with that for the time being and see if things change the next time I have to leave the bike sitting for a week.
But, for the time being, the crank/wait/crank method works fine for me.