I'm running 15.1 Premiere as of now, which is not the latest, but I've experieced this in the recent update as well, which is one of the reasons why I downgraded. In the middle of editing down a big, long video, audio waveforms just stop appearing on the clips I'm editng.
This has never happend to me in the v14 Premiere I've tried clearing the Madia cache clean, for adobe to make new peak files, did not help, reseting workspace does nothing as well. I short workaround I've found is bringing out the audio from within the nested sequence (I had to nest for production reasons) out into the outer most sequence and putting it disabled or muted under the clip. Then both audio clips have thumbnails with them, but only for so long. 10 more minutes of editing and those as well disappear.