How the heck am I supposed to get any development? I was doing okay for a little while, very very slowly building up towards 15, but then the black death happened and my provinces lost all development. And now, I'm pretty sure over a century later, I'm still at like 3 dev. They just constantly get little plagues getting in the way of bringing it back up. I can't even build clergy influence buildings, which are the only building I've found with plague resistance, because they require a certain level of development! And I don't think it's just me, all the AIs barely have any development either. The highest development in my game, in around 1370, is Milan at 23.
I'm assuming a lot of this is just that things haven't been balanced for the plagues yet, because they're from the patch released like a week ago. But ATM its pretty much impossible to do anything to grow a county's development, and thus to actually grow your duchy ("prince of") buildings