Are you using btrfs as filesystem and did you enable the use of snapshots as described in 11.7.2 of the manual?
The -x option in rsync prevents descending in other filesystems. Snapshots behave the same way, so the content will not be rsynced. On the other hand without the -x option you will get one complete data set for each snapshot as hardlinking does not work.
You would have to use something like btrfs send / receive.