Hi, it seems you had the same problem I had on my wk4, first of all the first time you should leave it for 24 hours on so you charge the battery. if that doese not work you have 2 choices.
1. to set it manually to 100% an see if it drops after that

2. Take it back cause there could be a fault in the CAPACITOR, which it would need to be change with one of Different Value.

that is all I can tell you about your problem.

Goog luck
Tony.