Useless motor vehicle maintenance plan

I purcased a maintenance plan with my Hyundai in 2016. This is the third time I experience the same problem when I take my car for a service. The maintenance plan does not appear on their system so I end up paying for maintenance related repairs. Though I paid for the maintenance plan and am in posession of a maintenance certificate. It seem someone did not do their job from the beginning. I have taken this up with them previously, however after almost 2 years this is still not sorted out.