Error on trying to expand data store on iSCSI volume

Hi, I have ESXi 6.5 running using a data set on a SYNOLOGY iSCSI Disk.

I expanded the iSCSI disk, but can not get the data set to expand.  ESXi sees the extra space, but I can't expand the current volume by selecting increase data store size- I simply get a message saying it can can not.  I can create a new data set.

I tried doing this while the machine was in maintenance mode

HELP!