Hi, deore 12 speed mech shifts from harder to easier gears no problem. However shifting down doesnt happen. I shift down, there is loose cable tension ( i can see this by pulling the outer from the shifter and seeing the cable is slack) but this doesnt feed through to the rear mech.i have played with b tension to no avail. High and low stop screws are correct

The rear mech when installed seems also weird. The top jockey wheel is pressed hard against the cassette, no gap at all until i put tension in the mech with a chain. Is that right?

Thanks in advance

Rust or crap inside the outer. The force will allow you to change up but the much lesser mech / slacker cable tension won’t allow the opposite. Outer and cable changing time.

Then adjust gears and sort the B Gap.

Funnily enough the same happened to me today, was going to take the Scott Spark out. Shifts up, but not down.
Must be the heat.
New inner and outer ordered….

Just installed a new inner. Then this started ☺️

Is the mech thing normal?

You do need to align the mech using the guide stamped on the inside of the cage according to what cassette you have. Instructions should be on si.shimano.com

broken the b-stop notch off your hanger?