The clutch fork can fail sometimes.  Unfortunately, the only way to verify this is to inspect the clutch fork itself.  To access the clutch fork, the transmission needs to be removed from the engine.  You can try and test the clutch fork by grabbing the exposed end out the top of the transmission and shaking it.  It should not move significantly, and if it does, the clutch for could be damaged, or the clutch fork could've popped off the pivot ball.  On rare occasions, the pivot ball can fail and allow the clutch fork to flop around.
FC3S Pro:  Problems, Components - Transmission Pivot Ball

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