These starters are a little fussy.
I always start with the battery and connections. Problems there will make you think it is in the bendix or starter.
Second is cleaning and lubing the starter drive gears. I seal the bottom vent and run a new vent up to the air box--much cleaner. Some put oil in the stator case. I have tried this but don't recommend it.
Other problems can occur with the bushings for the bendix, intermediate/crownwheel gear, bendix failures.
Finally there are starter failures. The pre 2013 starters are undersized. Brush and armature failures are common. I have had mixed success rebuilding. The 2013 starters are worth the extra $.
Slavens Racing carries all the parts. KTMTalk has lots to read on the subject.
KTMs--300, 518, 990