Reckon only a few things it can be. (see Peter's Starter motor problem)
1) Check your battery. Should be pushing around 20 amps minimum. This is a common problem with bikes (and any vehicle) if they're layed up for any length of time. May be insufficient power to fully engage the starter. Also check the solenoid - but this seems doubtful.
2) Possible weak spring in starter motor. Replace motor (expensive to repair I believe).
3) Gear(s) in starter motor in knackered. Best to replace starter.
Hope this helps!
Cheers,
Sid
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