I would use a file or Dremel on the offending tooth.

How can you guys call that clean?

Look at the derailleur cage and pulleys. Take a look at the rollers on the chain. The rollers are silver/gray. That's a layer of dirt/crud covering them. On a new chain? I'll bet that the new chain was not cleaned before installation, and it's rapidly picking up dirt from the derailleur pulleys and off the road. This much crud can lead to poor shift performance. What do you think the top pulley looks like? If full of crud, may be it's isn't rolling, and sliding as it was designed to.

If the cog tooth was damaged/bent before the cassette was initially installed, it could have been returned as defective. A little time cleaning the existing parts, and inspecting both the parts being reused and the new parts would have detected a defective cassette cog. After cleaning the existing parts, and inspecting the new parts to be installed, then installing the new parts, time should have been taken to adjust the rear derailleur and the shifter for proper functioning before the bike was put back into service.

Most mechanical problems on bikes are caused by the rider/owner, and are the result of inadequate mechanical ability, neglect or abuse. They are not caused by parts that suddenly go bad after installation, although parts do wear out and break even on the best maintained bikes.

Yep, OCD.

You can't even see the pulleys in the picture, one is out of the picture and the other one is covered by the chain.

The top pulley is easily visible, although it would have to be removed to see how much crud has collected inside the hub. If you can't see the upper pulley in the photo, you are blind. Besides, the time to clean and inspect the complete rear derailleur would have been while the wheel and chain were off the bike being replaced. Obviously that was not done.

There's more to a proper replacement procedure than just throwing new parts on. Cleaning and inspecting the associated parts when replacing damaged, dirty or worn-out parts is not OCD, it's good, competent mechanical procedure.

Check the hanger, might be a bit twisted too. As you come down onto the small sprockets the derailleur pivots backwards and the cage forwards which exacerbates the problem because the jockey wheel moves inwards relative to where it should be. So you get the indexing OK on the large sprockets and have problems on the smallest, or vice versa...

Look at the vertical axis through the two jockey wheels and cage from the back of the bike, it should be parallel to the sprockets. Turn the bike upside down and looking down from above check that the jockey wheels are running parallel to the sprockets. If not take the derailleur off and tweak the hanger with a shifting spanner...gently...

And no, that chain isn't that dirty...

Well I, for one, am amazed - that my signature Peter Mooney made it to 1000 miles. I doubt it has ever been as clean as the OPs rig. I guess I am deep into delusion or denial that it is still a great ride. Thanks RoadGuy for straightening me out. Though you have presented me with a dilemma tomorrow morning. Which bike do I take? Now that I have had my eyes opened, I know that none of my bikes has a chance at doing my 10 mile commute, let alone getting me back home.

Ben

The OP opened this thread asking what is wrong with his bike, and stating that the chain and cassette were new, leaving no other information.

Clean, Inspect, Lubricate, Adjust. For some reason when maintenance is properly done, many problems magically disappear. Without any further repair or replacement of parts. Funny how that happens...

Whilst checking out the grime in the picture, it does look like maybe that tooth is a bit bent...