You may need to lower your snapshot reserve (not only this volume but others as well) in order to allow for the free space to be used. On the Group information screen, take a look at how much you have reserved for snapshots. -joe
I have PS5000 with firmware 5.02 (yes, I know, need to upgrade) I have 380Gb free pool space in default pool I have a volume (with snapshot) that is 500Gb , but it needs extending Might be my old age, but I can not see an obvious way to do it in GUI  Seb
Expand volume on PS5000