Thanks to both of you for your feedback; nothing beats hands on experience.
My panels have the single knit Spandex.
I plan to start by a detailed inspection to try to understand where the membrane has slackened but since the “flapping” occurs at certain low frequencies I suspect the section with the longest vertical spacing, i.e. bottom part.
In your experience is it easy to identify the slackened area (s) and is it easy to see when the tension is corrected?
Alf