Someone has already answered this in another question, here is the text:

Yes, you can charge the contactless charging case like this, but I had to build it a bit underneath so that it worked properly (approx. 1 cm). Before that, the indicator LED was just flashing and there was nothing in the instructions about what that meant, so I guess the charging station didn't recognise the device.

So, as I understand it, you have to put something 1 cm thick underneath it....

Please do not use a metal plate or similar...

Thank you - but I have already tried that, then nothing happens at all, i.e. no LED that lights up or glows.