Since the 14.6 update I have been issues with waveform generation in my projects. I have already moved onto 14.7 and am still encountering the same problem. For those asking, I have remade the project in the most up-to-date version of premiere and the "automatic audio waveform generation" option is checked under my preferences.
Audio, Editing, Error or problem, Performance, User interface or workspaces
It's been around for a while, but I've only seen the problem with Multicam clips. The best I've been able to do is to use Sequence/Render Audio, and it will bring them back, but most of the time when I restart, they are gone.