Is it a SRAM mech? They're brilliant but incredibly fussy about set up. It's not like the 'do it by eye' shimano kit, you need to follow the (always excellent) on line guide and get all the measurements spot on.
Trying to get the odd hour in the shed to make progess so I can ride this on some dry trails!
This came on the bike I bought for parts. It was well used in some sandy conditions and left for many years. 😬