[Luthierking1]

Got a 2000 GSXR600. 30k miles, the last 12k in the last 18 months. Had a few electrical issues, now sorted.
Last few days, upshift is perfect but, downshifting, no 'feel' of detent. Doesn't actually downshift properly. End up stopped at junctions or roundabouts, whatever, stuck in fourth or third. Pull away, slowly (!), and eventually manage to get to an appropriate rev range and away I go, until the next junction.
Checked all the linkages and nipped up the clamp bolt on the gearshift shaft, all okay.
Any ideas? Anyone else had this problem?

 

[SparkyMJ]

If you put the bike up on a rear stand, can you shift the lever through all 6 gears while jiggling the rear wheel back and forth? Or is it only when you're riding?

If it shifts fine with the rear tire off the ground, then you might have a clutch issue instead.

Was the bike ever dropped/crashed on the left side? Sometimes incidents on the left side can force the shifter, causing internal damage to the transmission. I've seen this happen, and I've ridden those crashed bikes, they just don't shift normally ever again without rebuilding the transmission.

I know you said you checked them, but no binding in the linkages? Sometimes shifting in one way (up or down) being easy is just a bind in the linkage not fully allowing movement in one direction. Ensure that any ball joints or hinges are free moving, lightly greased, and clean.

Hope this helps.
-Mike