I've been having the same kind of issue actually. When using CUDA, if I apply effects and try to play back, it either does not play at all, or I get a white/blank playback screen. I have to actually turn CUDA off to get full support. Any ideas on what could be causing this?
Quick post - 1 idea.
Not all graphics cards are Adobe approved. If they aren't in a text file as approved, Adobe may not use the GPU's. At least as of about 2 years ago.
What card do you have?
Do a quick search on "adobe cuda hack" or similar. Here is one thread discussing ot, although I am not familiar with this one, and don't know how clear it is:
https://forums.adobe.com/thread/629557
I used this web site when I upgraded my video and tuned my machine for CS6 about 2 years ago. I amm not sure how dated it is, but there is a lot of good information to help you understand how Adobe used the GPUs. Plus I think theyu have/had a script to automate the :hack.:
Good luck!
Michael