Something else to try... leave kill switch to off.. hit the starter.. then move kill switch to run...
Why? sometimes (particularly when hot), the bike fires just before top dead center and the system struggles to get it over center.. using the kill switch method gets it rolling first... even if this helps, keep in mind that the system is probably not up to par (battery, connections, starter, etc.) because you shouldn't have to do this..
I went through what was said in both previous replies and both are good advice.For a couple of years though the troubles came and went. I finally had enough and replaced the cables, solenoid and starter. I made the cables from components bought at an auto parts store and used a universal solenid from there as well with a blade the fuse to replace the dogbone. The starter came from dbelectical.com, but there website doesn't include them anymore. Quite a shame as they were good quality and very competitively priced.
The Achilles Heel in my starter was the comutators needed to be resurfaced and the machine shop charges would have been more than the new starter. Refreshing the starter as described in the available how to with new brushes will probably get her back to normal at least for a while. If problems keep occuring then IMHO the long term solution is a new starter or a rebuild at a reputable shop.
I have had success with cleaning the commutator with 400 grit sandpaper. Unless the commutator is badly out of round that will work. An out of round commutator will cause intermittent brush contact with the segments. I have used a starter that had one segment of the commutator burned along one edge from brushes that were too short.
Thoroughly clean out the carbon dust. Apply fresh grease to the gears and bearings. I was able to get new brushes, I believe from the dealer.
Check all the grounds on the bike. make sure they are all clean and tight.