What are you actually trying to render, the Frost objects or Krakatoa particles? Why is PFlow involved at all?
I’m not exactly sure why you are using PRT files as your particle sources. Why not just use a normal Birth operator and a Position Object to put the particles on the surface of the car? Either way, its possible that the PF Source 001 System Management -> Particle Amount: Upper Limit spinner is preventing all of your particles from being birthed at once. By default it limits the system to only 100 000 particles so if your combined PRT sequences has more particles it would just stop loading part way.