[Closed] Rear mech not shifting down very well...can the spring weaken?

you mean shifting "up" into smaller sprockets (but higher gears)? IE a conventional, not rapid rise mech?

I'ev not seen this cause but have heard of it.check all the mech pivots, including the top one, and B-tension adjustment.

Sram mech by any chance?

Sorry, yes shifting into smaller cogs. It's an XT mech but I've had it on the bike for quite a while.

Can, and does happen to older mechs. Prime cause of slow upshifts is usually stiff/dirty/damaged cables, have you changed both the inner and outer? If so, then it's likely to be the mech (or how it's set up, see Al's post), or a badly worn drivetrain.

As Al said this can happen but I've never seen it either. Check every adjustment possible on the mech. Have you been running big ring, big ring? This is supposed to have the rear mech at its full extension and this can bugger the spring

Worn jockey wheels can cause this too - not just ninja stars but excessive side to side play too - usually comes with ninja stars though)

Don't really ride big ring-big ring and I changed outer and inner cables which feel much better than before. Will maybe take the mech off the bike and have a proper look at it. Have never done that, so maybe it's had a knock or just got a bit seized from years of crap weather? Failing that I suppose it's shiny new mech time?

Brake cleaner and then re-grease.

dirty cables

Very common on SRAM, never had it on Shimano.

Also check the hanger is straight and what Alpin and ndthornton have said.

+1 for dirty cables causing drag. The Mech spring has to pull the slack cable through that the lever releases as you change to a smaller sprocket, if the cables aren't running freely enough it'll struggle to do it. You'll be able to tell if it's the mech or the cable easily enough if you undo the cable from the mech and try moving the mech by hand .

I had it once on an XTR mech.

Edit: the spring wear I mean