I had a similar problem and adjusting timeline didn't work. By chance, because of a launching issue, I found something that may help (it did the trick for me).
After shutting down the app, use Task Manager to see if PD is still running as a background process, and End Task if it shows up. If it's running, it can even prevent a fresh launch, which was the issue I was having. Ending the task made the waveforms appear.