I plugged my Osmo Pocket into an ikea USB unit last night, and went to bed. 9 hours later the battery of the OP is just at 54%.

The power from the ikea USB device is 5v and 2,4a so it should be more than enough. And the op is turned off...

What could be wrong ?

Hello and good day. I am sorry for the trouble and  thank you for reaching out. With regards to this concern, you may give a try to use a different wall charger, and also you may try to use a different USB Cable just to isolate the issue. If ever the issue still persist we would recommend sending it in to the facility for further checking of the issue you can go to this link https://www.dji.com/support in order for you to have the guide on what you'll need to do. Please keep us posted on the said issue. Thank you.