I think I know exactly what happened to you.
Here is diagram
P400 (P420i) with 12 Drives of 3TB --- Single virtual Drive of Raid 6 30GB --Shown to -> Vmware ESXI
Meaning, you have entire 12 Drives grouped ( (12 -2) X 3) in HW controller as single Virtual Drive of 30 GB.
Of course , Vmware can only see single Lun ( LUN = Virtual Drive from Controller). This is why you can
resize it, but you can not add more lun/drive since there is none.
There are many ways to config HW raid controller. Say you want to have 2 LUN/Drives on Vmare,
you would do 2 virtual drives in Raid OS.
For performance reason, you want to stay with SINGLE virtual drive.