USB redirection not working

We have a Xenith client that connects fine to the XenApp 7.6 desktop but we cannot get USB drives to redirect.

We have made sure the drives are FAT32, the thin client sees them and USB redirection is working from a laptop.

Our xen.ini file contents are:

;*************************************************************

;*                                                           *

;*          This xen.ini file was generated with the         *

;*            Configuration File Generator 7.3.03            *

;*              Copyright by Thomas Moellerbernd             *

;*                                                           *

;*                 https://technicalhelp.de               *

;*                                                           *

;*************************************************************





;*************************************************************

;*                          General 1                        *

;*************************************************************



autoload=0

Privilege=None

AdminMode=yes



;*************************************************************

;*                           Network                         *

;*************************************************************



SignOn=no



;*************************************************************

;*                       General Session                     *

;*************************************************************



SessionConfig=All MapDisks=Yes UnMapUSB=Yes



;*************************************************************

;*                             ICA                           *

;*************************************************************



SessionConfig=ICA

MapDisksUnderZ=Yes

USBRedirection=HDX

What are we missing?

Thanks