This a common problem with most bikes, The starter switch is the weak link. I have one on order for a friends 650 bergman right now.
Try opening the housing containing the switch (two phillips screws) and spraying WD 40 or such.
Most of the time it is mechanically binding, My friends parts are too far gone to just spray. Had to order the whole switch housing just for the switch inside. it cost $102.
That is why I carry a mini can Of WD40 with me on the road. I have done this on hondas suzi's, harleys, yamahas, and one bmw.
Hope this helps.
MaxB ( who has done this on a bunch of different bikes.)