Question about Walter Wallet single dock

With my iPhone X, it doesn't always charge reliably either (without a case). Before I go to bed, I put my iPhone in the charging cradle and it shows that it is charging. In the morning, I pick up my iPhone and see 40% battery left, which means it hasn't charged properly. If this happens more often, I'll probably have to send it in under warranty.

I own the iPhone X and my wife owns the Samsung S8. With both of them, it only works without a mobile phone cover, although both covers are Qi compatible and work perfectly in our car, for example.