I've plateaued on my most modern games which are also the most technically challenging. Part of that is because I've had a pretty good sized collection for a few years so I'm always playing different machines throughout the week. Now, however, I've decided to focus a bit more on a just few.
I don't want it to feel like work, as though I'm training for something. I want it to be fun, so I add an extra ball (4 instead of 3), install an easier mode if that's an option or tweak features like various ball save times, and tighten the out lanes. This gets me past my plateau, and makes a game feel really fresh again. After doing this for a while, I can see how going back to the factory settings will also feel fresh and become more fun for knowing better where it all leads.