Baffled by 11 speed Ultegra and Dura Ace Cassette
 

This is driving me crazy. I am using a brand new Shimano Ultegra R8000 11 speed cassette and it works fine. I tried replacing it with a Dura Ace 9100 and the last cog won't lock into place. It is if the new cassette is wider than the Ultegra which doesn't make sense as they are both 11 speed. What am I doing wrong that it won't work?

Video of the Dura Ace 9100
Video of the Ultegra R8000

Is there a spacer you can remove? Assume you have 11 speed freehub.

Thanks shelbyfv. Yes, there is a spacer for the Bontrager hub that has a 11 speed body and it is used for the stock Ultegra. The bike is a brand new Trek Domane. I can take the spacer off but that doesn't explain why the Ultegra works and the Dura Ace doesn't.

If the R8000 cassette has a 34 tooth large cog, it will fit a 10 speed freehub body. That is why the spacer would be necessary on an 11 speed freehub. The Dura Ace is 11 speed compatible only so the spacer will have to be removed.

What is the thickness of each cassette in mm in the splines?
What is the length of the freehub body in mm along the engagement area of the splines?

If the biggest cogs on a cassette are big enough, the cassette can be designed so that those cogs can sit out over the wheel's spokes rather than come all the way down to the splines on the freehub body. This allows a wide cassette to fit on a narrower freehub body (i.e. an 11-speed cassette on a 10-speed freehub).
This is standard practical for MTB stuff these days... plenty of MTB cassettes are wider than the freehubs they sit on.

But if you have an 11-speed cassette that can mount on a 10-speed freehub, and you stick it on an 11-speed freehub... the part of the cassette that contacts the freehub is wider than the freehub itself, so you need a spacer to fill the gap.

Thanks everyone for your collective wisdom. The spacer will come off and I now understand why.