I have had this problem.
There is a safety latch that engages the plug to prevent you from removing while charging. This can fault. There is a youtube video that describes how to replace the part. He also describes that the switch or sensor may be sticking and that you can tap the car body...
Just to be clear -
PUSHING THE BRAKE PEDAL RECHARGES THE BATTERY.
If you need to slam on the brakes such that the friction brakes engage, you were going to do that with or without Regen engaged.