It didn't work for me either at first, no matter what I tried.
It finally worked with this:
Leave the pen to charge for 4 hours. Then move the pen back and forth a little in its normal charging position (at the back of the housing, see instructions), perhaps making small circles.

I thought so too, but then I realised that I hadn't switched on the Bluetooth, and the pen has to be inserted in the right direction at the back.
Maybe this will help you

Read the instructions, works perfectly for me