Chain jumps, slips, crunches, is dropped - checklist

The reasons for the above behavior are often due to wear on the chain, the sprockets, or both – but not only. The following checkpoints will help solve the problem:

• Chain wear inspection with a Chain gauge or a caliper
• Check if the chain has stiff links. The reason is usually a lack of Care.
• Check the rear derailleur for mechanical damage (e.g., bent derailleur hanger).
• Check the cassette and its sprockets for wear and fastening.
• Rear derailleur settings check:
  
  
  • Chain jumps over the first or last sprocket:
    The derailleur's alignment or limit stops must be adjusted here using the adjustment screws «H» (for higher speed - smallest sprocket) or «L» (for lower speed - largest sprocket).
    
    many articles about how to properly adjust a Shimano XT derailleur. this video track.
    
    Rear derailleur adjustment tool Line-Up Accurate – Laser Alignment Tool
  • The chain jumps to the lowest cog.
    If the cassette is compatible with the derailleur, the lowest cogs may be loose. Tighten the cog screw to the correct torque. This is usually indicated on the cog screw itself.
  • Chain jumps within the gears:
    The tension of the shift cable needs to be adjusted here. For this, there is usually an adjustment screw where the shift cable enters the shift lever. If the chain struggles to jump into higher gears when shifting, turn the adjustment screw counterclockwise. If the chain skips gears, the shift cable needs to be loosened slightly. Proceed in 1/4-turn increments, checking each time until the gears shift up and down cleanly.
  • Chain grinds on the smallest gears (large sprockets) / The chain slips:
    The loop (so-called B-tension screw) needs to be adjusted here. It controls the distance of the upper jockey wheel to the sprockets. A guideline for the distance is approximately 1.5 times the length of a chain link. It is best to follow the derailleur manufacturer's instructions.

A good troubleshooting list can also be found here.

Instructions and a video for the Shimano Di2 drivetrain can be found under SHIFTING - SHIMANO - ‚Setting the Di2 rear derailleur'.