How big is your VM? I’ve found that I must call fly scale memory 1024 before my apps will spin up. Elixir apps can start up without increasing memory, but I’ve found this only really works with an “empty” phoenix app. The initial 256Mb provided is just a little too small for anything more than that.
I’ve also found that my apps don’t generally use more than 400Mb, so 1024 feels like overkill for me, but we can’t scale to 512 so… 
Still, 1024 allows for lots of useful ETS tables! 