8011 Diff stuck

I have an 8011 and the diff won't press down,
The linkage is free enough and the rod comin out of the back end moves very slightly so it doesn't look seized.
I am very mechanically minded and Bought the tractor as a none runner last year and only noticed this.
I did replace the brakes, maybe I missed something when I had the half axle off?
Any help would be appreciated, thanks.

The dog clutch only engages when they are rotated to the right position so you have to keep presssure on it till it engages.