Danmed

I'm new to Drivepool so apologies if this is an obvious question.. I have 4 3TB discs set up in a single pool, and i've installed the addon to "order file placement" the files across them. I'm not interested in duplicaitng files anywhere, i simply want my files in order spread across the 4 disks.. When i write new files to the pool, it's putting them on the 4th disk, and not the 3rd disk (which isn't yet full).. The write speeds are also very low, around 15MB/s whereas when i was initially filling this pool up, they were over 100MB/s I thought that as i wrote files, they would automatically go to the correct disk, not be written to one and then balanced afterwards...I am also confused by my suddenly slow speeds.. in the screenshots below, you can see a little blue indicator on the 3rd disk which reads as "un-duplicated target for rebalancing"... i'm not sure why that's there but is that what's causing this issue? As the data is being written to disk 4, that blue marker on disk 3 is moving.. why on earth isn't the data just written to the 3rd disk in the first place?