ultegra 10sp di2, slipping

Hi, in the small ring, not the granny gear on the cassette but the cog next to it i’m having problems with the chain making lots of noise in it or slipping off onto the next sprocket, is this an easy fix or is it time for a new cassette?

I’ve had the same issue looked at 3 times by my lbs but they seem to struggle with it in a work stand finding the problem.

It’s also really loud in the middle of the cassette in general???