Unable to use last 830GB of 10TB space despite balancing

Hi

I have an ReadyNAS 314 with 4x4TB HDD in X-RAID2 (RAID5).

According to the System->Volume page I have 836.36GB of 10.09TB free.

However whenever I try and copy anything to it - I get an error message that it's full.

Via putty:

Last login: Wed Jun  5 14:34:32 2019 from 192.168.1.48
root@ReadyNAS314:~# btrfs fi show
Label: '7c6e00a0:root'  uuid: 675aefad-8ae6-4610-93a1-adc036463976
        Total devices 1 FS bytes used 573.24MiB
        devid    1 size 4.00GiB used 1.64GiB path /dev/md0

Label: '7c6e00a0:data'  uuid: c04fc549-7c2e-4ede-938b-8fc620798636
        Total devices 1 FS bytes used 10.07TiB
        devid    1 size 10.90TiB used 10.90TiB path /dev/md127

root@ReadyNAS314:~# btrfs fi df /data
Data, single: total=10.88TiB, used=10.06TiB
System, DUP: total=32.00MiB, used=1.19MiB
Metadata, DUP: total=11.00GiB, used=10.49GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
root@ReadyNAS314:~#

This would I think suggests it needs a data balance. However I have run:

btrfs balance start -dusage=90 /data
btrfs balance start -musage=90 /data

twice now and it's not made any difference.

Any suggestions??
Thanks

Andy