Looking at the syslog, it appears there's a BTRFS error. That would explain why it isnt converting.
Dec 30 08:09:37 cortex ool www[13111]: /usr/local/emhttp/plugins/dynamix/scripts/btrfs_balance 'start' '/mnt/cache' '-dconvert=single,soft -mconvert=raid1,soft'
Dec 30 08:09:37 cortex kernel: BTRFS info (device sdb1): balance: start -dconvert=single,soft -mconvert=raid1,soft -sconvert=raid1,soft
Dec 30 08:09:37 cortex kernel: BTRFS info (device sdb1): relocating block group 2442826612736 flags data|raid1
Dec 30 08:09:37 cortex kernel: BTRFS warning (device sdb1): csum failed root -9 ino 299 off 969187328 csum 0x3d995087 expected csum 0x1930d4f7 mirror 1
Dec 30 08:09:37 cortex kernel: BTRFS error (device sdb1): bdev /dev/sdc1 errs: wr 0, rd 0, flush 0, corrupt 887, gen 0
Dec 30 08:09:37 cortex kernel: BTRFS warning (device sdb1): csum failed root -9 ino 299 off 969187328 csum 0x3d995087 expected csum 0x1930d4f7 mirror 2
Dec 30 08:09:37 cortex kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 0, rd 0, flush 0, corrupt 726, gen 0
Dec 30 08:09:37 cortex kernel: BTRFS warning (device sdb1): csum failed root -9 ino 299 off 969187328 csum 0x3d995087 expected csum 0x1930d4f7 mirror 1
Dec 30 08:09:37 cortex kernel: BTRFS error (device sdb1): bdev /dev/sdc1 errs: wr 0, rd 0, flush 0, corrupt 888, gen 0
Dec 30 08:09:37 cortex kernel: BTRFS warning (device sdb1): csum failed root -9 ino 299 off 969187328 csum 0x3d995087 expected csum 0x1930d4f7 mirror 2
Dec 30 08:09:37 cortex kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 0, rd 0, flush 0, corrupt 727, gen 0
Dec 30 08:09:38 cortex kernel: BTRFS info (device sdb1): balance: ended with status: -5