I have a newish full suss, I cannot for the life of me get the rear derailleur to work properly.

I moves up the cassette (against the spring action) perfectly but when I move down the cassette to the smaller cogs, relying on the Der's spring action gears are often not selected. There is a click at the trigger but no gear move, I try again, a vague click and perhaps still no move then a couple of pedals later I get two or three gears at unexpectedly.

I have tried different techniques, easing pedal pressure etc and also followed all the guides for setting cable tension etc.

The set up is Deore shifters and SLX rear mech, the cable has been replaced bt a LBS without improvement.

Presently I'm missing my thumb shifters, never let me down! Anyone got any advice?

Sticky cables, get yourself a new LBS.

is it a new chain – give it a good degrease and re-oil – that stuff they ship them in can cause problems.

other than that – sounds like a cable problem – what happens if you disconnect the cable and move the mech by hand? If that works fine then def cable.

Could be poor set-up in lots of ways, cable tension, problem with cable (kinked etc), cheap cable, b-tension screw to name a few.

Never seen OEM chain lube cause problems in years of LBS work

has it ever worked properly? (you say newish)
is it a subtle bend in the hanger or derailleur?
is the cable outer perfect for length in the workstand but not quite long enough when the suspension is compressed?

Thanks for those suggestions.

I have removed the cable from the mech and there is more resistance than I would expect when holding the cable and moving the shifters. The cables all look a bit on the cheap side. I'm gonna replace them.

Which gear cables are the best?

I've always liked Shimano XTR cables. Trying out some Gore ride-ons at the moment on the winter bike and the jury is still out.

XTR every time for me!

Sort of fixed it. Couldnt find any XTR cables local and wanted to sort it over the weekend. I replaced the whole cable with whatever the LBS had on a roll and tensioned the cable right up whilst on the smallest cog.

The gears now work much better, I would say only one in ten changes down a cog sizes doesnt work reliably. Still think the spring should pull harder though.

Cheers.