I have exactly the same problem !! The batteries in the power boost grip are fully charged as indicated by charge lights going off and the charge indicator on the camera. If I then do not use the camera for a day or two the batteries in the boost show as discharged. If I remove them and immediately reinsert they now show as fully charged. However if I immediately plug the charger into the grip the charge lights are on for over an hour.
This is causing me major uncertainty as to what state the batteries really are. Is the problem a charging issue, a camera reporting issue or a battery defect ??
The batteries are the NPW126S model.