After running reference many times, Backstop starts to fail until computer restart

macOS 10.13.1
MacBook Pro (15-inch, 2017)

I've been learning Backstop, so I've been running a ton of backstop reference a lot. Twice now, reference has silently failed to run all of my scenarios. I don't get any sort of error in the log, and it says that reference has completed, but when I look at the actual images, not all of my scenarios/devices will be there. The only way I've found to remedy this is to restart my computer.