DP data showing as "Other"

Hi there, having a little issue.

Recently when balancing has been triggered, not all of the new files added to the SSD are being moved off into the rest of the pool.

See that gray area I've circled on the SSD drive? That's data that's part of the Pool. I'm not sure why it's showing as "Other" and why it isn't being moved off the SSD.

I'm using the SSD Optimizer and it's been working fine until the last few days.

Help!

Thanks

R