Is your volume status showing as redundant?
If so, please try a reboot. Wait 5-10 minutes and if expansion hasn't started try another reboot.
If that doesn't resolve the issue there are some additional suggestions here: What To Do When Expansion Doesn’t Start
X-RAID2 volumes are expandable. They automatically expand, however sometimes a reboot is necessary before expansion can take place especially when doing vertical expansion (replacing disks with higher capacity ones).