Thanks for sending the PSD. It's starting to make things more clear.
I cannot reproduce the problem here with your file either with Photoshop CS6 (or Photoshop CC 2014.1), which implies that the problem isn't systemic to Photoshop but a specific issue, possibly with the GPU driver on your machine (Photoshop CS6 was the first version to accelerate the Liquify filter in the GPU).
A key clue is that your sketch is on a transparent layer. There would be no transparency for Liquify to deal with if you flatten the document such as would happen if you work on a JPEG.
Perhaps the transparency is somehow confusing the GPU-resident Liquify software on your particular system with your specific hardware and/or display driver..
There is a way to start Liquify and run it only on the CPU, instead of in the GPU... You could try it. Hold the Alt key down (I guess that would be Option on a Mac) when starting Liquify. You will see a thin dark outline around the work area in the filter if you're running it on the CPU and it may seem to run more slowly. I'm curious whether this can be a workaround for you.
Is your Photoshop CS6 fully up to date?
-Noel