In Crystal Cove, there's a point where kobolds won't go any further after they've been through once. On the bottom left (Southwest), at the entrance to the bowling alley, right where the water is - if they've gone through and collected a shard once, the next time they come, they'll turn around at the last torch before the water. This has been confirmed by multiple people on multiple occasions (saw it standing there on 3 of my last 4 runs), and it's costing hundreds of shards, since there are many purples past that point which never get picked up. Re-placing the torches seems to fix it, but you have to have seen 15 workers turn around to know where the problem is.

For both of you still running CC, the non-changing loot isn't the only thing keeping people away. Getting tired of having a good-looking run, and then the kobolds waste 10 minutes walking down there and coming back.