Before you maybe wreck it with glue.
Try using a thin strip of wax paper inside the ball mounting.
A small piece will fill up the gap causing the slippage without screwing things up on you.
If it starts slipping again in a couple of months, just replace the paper to fix it.
2012 Focus Titanium w/handling package, stick shift, MFT, Sony radio, Moonroof in Tuxedo Black.
2012 Lincoln MKZ Hybrid (wife's car) all the bells and whistles also in Tuxedo Black.
2007 Harley Davidson FXSTC Softail Custom, in Vivid Black.