Discount on Toll Charges

I think the discount on motorhome charges on the Tolls is a fantastic idea and we have used it a couple of times when on the M6.  However I was driving up to Sutton Coldfield last weekend to go and pick up my husband to travel on to the Lincoln Motorhome Show.  When I got to the booths off the T3 junction there wasnt anyone to show my Membership card to and so I was charged £10.  Has anyone else experienced this?  Does anyone know what we have to do if the booth is automatic and not manned? 

Thank you for your help.