Bike is only about 4 months old. I was indexing my gears last night, and the rear mech cage is getting stuck when shifting from largest cog to second largest cog. If I shift to the third largest cog, it will pull it down. Shifting from second largest to largest works fine.
I've played around with the cable tension, etc.
I tried pushing it inwards without the cable attached and the same still happens but it eventually pulled its self back in.