If you have had no vouchers at all, firstly do you have conventional key/card prepay meters or are they smart meters?
If smart, you don't get vouchers. The credit is applied automatically.
If conventional, do E.On have all your correct details? Do you have the app or online account and are your details all correct?
Several cases have been flagged up where folk just top up a meter using the key or card but have never actually registered with the supplier. Often the case where you move into a rental property with existing prepay meters. I did that twenty years ago...lived in a flat with a key meter and just took the key to the post office every month and stuck a tenner on it. Didn't know I had to register with the supplier! Had I done that, they could have removed the existing £600 debt from the meter and I would have saved a fortune!