Never had a problem with my XT’s, 1000’s of km’s on them … but I have seen it happen.
– remove the arm (and the bolts from the arm) and clean everything!
– clean it again
– check the splines, if they’re worn it’s junk
– apply a decent waterproof grease everywhere where metal touches metal
– fit the arm, make sure it hits home
– preload the bearings (just a small amount of drag, no “gritty” feeling)
– tighten the allen bolts alternating a quarter turn at a time
– when snugged up use a torque wrench for final tightening
– nip up the preload
All good apart from the last point.
No point nipping up the preload, it wont do anything. Once you’ve set the preload prior to tightening the arm, leave it alone.