I am actually in the process of posting something involving this. Mine started doing the same thing I think. Does yours stop, kind of as if the starter doesn't have enough power to turn the engine over on the compression stroke? That's what mine does. I just let off the starter button and hit it again and it works fine though. But I'd still like to address the problem before it gets worse, if it gets worse. I thought it was the battery at first, but I've been riding it so if the battery was low it should be charged now.
Also, if I adjust my valves it seems to change. If I tighten the valves I don't get that drag anymore, but it takes more cranks to start, which I'm sure isn't good. I'm wondering if it has anything to do with the decompression mechanism? Maybe it's not opening on the compression stroke and the starter is having a hard time turning the engine over? I'm not sure. If you make any progress let me know.