WD15, support for 3 monitors

Hi,

Bought a second hand DW15 docking station for second workspace. Checked it has the latest firmware, which it does.

Setup as follows:

- BENQ GW2270 - HDMI 

- DELL U2312HM - mDP to  DVI (Have tried mDP to DP)

- DELL U2312HM - VGA 

No matter which way I try it, one of the DELL always comes in at 640x480. The other two are 1920x1080. 
Also the one of the DELL monitors is not detected on the VGA.

 Is this a limitation of the WD15?

The same system connected to a newer docking station works fine. Any thoughts?