Set up ;
Single fron ring
Rear cassette with 42 t extender
Goat link with extra long B screw.

Issue;
Will not shift into 42

Tried;
Removed inner cable and pushed mech up to check limit stops goes up to 42 fine and drops back down fine.

B screw adjustment looks fine with 5mm between top jockey and 42

Cable back into mech and nipped up / does not shift into 42. Inner and outer cables are near new.

Mech hanger looks straight what else have I missed could I try ?

Thanks

Does the cable pull tight in the gear below? Can you pull more cable?

if the mech goes all the way over without any issues up to the stop then my best guess is the issue is with the shifter.

It does appear that when trying to shift into 42 the shifter lever cant quite push enough.

When in 36 cable is tight in rear mech with a little pull to it if (wiggling it) tech term

how many gears on the cassette and how many is the shifter designed for?

1 X 10
SLX 10 spd shifter

Sounds to me like you’re “one whole gear out of sync”.

i.e. your shifter is in gear #9 when your mech is in position over gear #10 on the cassette, consequently when you shift down to gear #1, your mech only goes to position #2

What would be causing that Ecky

cable too tight?

Release the cable, set the limits, do the cable back up just tight on the smallest cog, you can always use the barrel adjuster to take up a little slack.

What would be causing that

… not releasing the shifter all the way to first position before you started

Thanks all took it all back and with a slight tweak to limit screw now goes into 42.