Okay, so I just went through this, on a 120TB setup. Drivepool was unable to balance and duplicate properly throughout this. Unfortunately the StableBit teal (while involved thoroughly, had trouble figuring it out.)
I found two problems.
Problem one: I had a failing hard drive. Once I found and removed that, DrivePool was able to scan, and tell me a file it was failing on.
Problem two: Many times, it was an old file that the file path for some reason was being detected as too long. I usually would go through each drive individually, and either delete, or remove from the DrivePool. Then when you measure again, it will find another file, ect, ect until you are good. I eventually found two directories, and removed the two directories, and all came good again. I verified the integrity of the files, let DP work its magic getting caught up on duplication and such, then place the directories back, in a shorted folder structure.
This solved the problem.
Hope it helps.