I recently acquired the latest 10spd X9 derailleur (Type 2.1, long cage) and it’s not working OK with an XT cassette (11-36).

I first noticed something a bit odd when I set up the limit screws. Basically, the rear derailleur is fully extended when aligned with the largest cassette sprocket, i.e. the limit screw isn’t what prevents it going too far inwards.

At first I thought this might be OK, but after trying it today during a 3 hour ride, I noticed that the rear derailleur is getting stuck in the lowest gear. If I click down, nothing happens unless I go over something rough enough to ‘shake’ the derailleur back into action.

So two questions:
1) Why is the derailleur maxing out on the largest sprocket when it’s the 10 speed version ?
2) Can the return springs in SRAM derailleurs become ‘stretched’ after a short time such that the derailleur won’t move down to smaller sprockets ?

The derailleur hanger is fine before anyone asks – the bike had been running perfectly on 9 speed before this.