I'm the King of Wales, and I've owned Venice and the Isle of Mann for approximately 150 years and there is still no de jure kingdom drift happening to them. By this point they should definitely be part of de jure Wales, but I'm still fighting off Scotland for the Isle of Mann and the Kingdom of Lombardy for Venice. Shouldn't they have experienced de jure drift by now? They haven't even begun the countdown.
I see that other kingdoms are experiencing de jure drift just fine, but no holdings I ever gain do.
Are you holding the whole Duchy of Galloway, and are you holding the King of Venice title? I have no control over de jure drift mind you so whatever the problem is has nothing to do with CK2+.