>>1300023In no particular order:
1. Clean your entire drivetrain. You can't expect a dirty drivetrain to work properly.
2. Is your drivetrain worn out? Chain too stretched? Chainrings and cogs worn? Replace what's worn.
3. What was the last time you replaced shift cables? Old cables and cable housings get sticky and cause shifting problems like this.
4. Remove the chain and disconnect the cable from the rear derailleur and move it through it's full range of motion by hand (including the motion of the knuckle). Is it sticky at all? Clean and lube it (use something penetrating and for precision mechanisms like Breakfree or similar) or replace the derailleur with new.
5. Derailleur loose? Tighten it. Derailleur hanger bent? Straighten or replace it.
6. Is your shift lever working properly, or is it sticky? Pull the rear wheel off so the derailleur is free to move, and check the action of the shift lever; does it seem sticky at all? Clean and lube it, or replace it with a new shift lever (I've even had this problem with Shimano Ultegra shift levers)
