The Prusa has developed quite a clicky extruder motor today. It will lay filament down fine for a while, maybe the first five minutes of the print. Once the clicking starts, you can see the gears up above slipping through the window and it either lays down the filament unevenly or not at all. It did this with multiple PLA filaments so I put an “Out of Order” out there.
Edit: upon further tinkering, thinking it might be the filament after all. Doing fine on a much smaller print now. Removed the out of order tag, but someone who is more familiar with the printer might want to be on the lookout for clicking.