Why does this invisible wireless charger not work？

Please use the wall adapter which came with the package. The adapter matters to its performance. Please do not use a USB port without the adapter.

Is the black pad sticker is necessary?

It's necessary when the invisible wireless charger is under the desk. It's functional and help you locate the charging spot. If you use the charger on the desk, no need to use the black sticker. Please make sure to use the original power supply.

Why does it not work with a popsockt or otterbox defender case?

Please make sure to use the wall adapter which came with the package. And the phone case should not contain any metal things. It would interfer charging and the charger would stop charging.