6MT wont shift?

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you sould have clutch drag. where the clutch is not releasing so your syncro is not spinning fast enough for the gears to engage.

though this is an OEM clutch w/ low miles i would not expect thing like this to happen.

its highly unlikely the throwout Bearing is out. because if that is the case, he wont be able to engage the gears while the engine is not spinning. its also unlikely that the master or slave cylinder is out, because he wouldnt be able to get the gear inplace when the engine is off (the clutch MC / SC does not require any additional power supplied from engine unlike our brakes). its also unlikely for it to be the fork because he also wont be able to shift at all.

I would check the flywheel, clutch disk and pressure plate for witness mark of rubbing. if the clutch looks clear, than I would suspect your syncro is the problem.

Note: my experience came from toruble shooting with ACT prototype clutch dragging with my BRZ. Ive suspected everything, at the end it turns out to be the ACt clutch (@ approx 20K miles)