Only my humble points,
So assume its a double on the crank, & this only happens when you are in the smaller chain ring ? there re no issues with the FD ? & obviously the cassette ring is not in any way slightly bent, I don't know Sram one assumes you looked at the teeth profile closely ?
If you take the chain off, does it index through the range on the shifter smoothly.
When you say it its hesitant ? does it ride up onto the 7th then drop back i.e. not going far enough over ?
Hi. Thanks for your reply. To answer your questions:
1. chain ring: It happens for both small and large chain ring.
2. FD. FD is not an issue. No rubbing as long as it's not cross chained.
3. teeth profile: They look okay. We didn't see any damage or missing tooth. But swapping to another cassette may be something we would try next.
4. index through the range on the shifter: Not tested yet.
5. hesitant: It does not go far enough. Once it goes over with a little extra cable pull or pushing the RD with finger, then the chain stays on 7th cog quietly.