Here's a possibly different explanation for your difficulty: your grip on the G19 isn't as strong/hard/tight as it should be in the first place.
Here's my reasoning: when you grip the G19, the size fits you so well and makes you feel so "locked in" that it gives you the illusion of gripping the gun as strongly as you should. The bigger size of the G17 doesn't "lock" your hand in as much and your "weaker" grip on that gun is giving you difficulties. There are quite a few threads on how to grip here that should give you some good pointers.
Without seeing you shoot, it's near impossible to diagnose what is going on, so I could very well be wrong in my attempt to diagnose what's happening with you. It may just simply be that the ergos are so different for your hand that the G17 will never work for you as well as the G19. Still, review the threads on gripping the pistol and start applying them to the G19 and 17 and I think you will start seeing improvements on your G17 shooting (as well as with your G19).
In any case, my suggestion is only worth what you paid for it :-)