Rear Derailleur - getting stuck in largest rear cog 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.

Any ideas?
On my bike I need to be on one of the smaller front cogs for lower gears to work correctly. If I'm on the large sprocket lower gears are unreliable due to the bend in the chain. My owners manuel said not to use extreme combos eg large front to large rear