SRM7: PO Webdynpro fields not saved

Hi,

We've added customs fields to the PO header in SPRO using the guide here Custom Field for SRM SC WebDynpro Application

We've also added the fields using SE80 to /SAPSRM/WDC_DODC_PO_H_BD1

They are appearing ok on the purchase order screen but when we enter in values they are not saved. (switching tabs resets them)

What other steps are we missing?

Thanks

Tony