Thanks for all the comments so far; I will go through them in more detail in coming days.
For greater precision, I'm in temperate Ontario, with modest sun, rain, snow all making an appearance. No salt, and not the burning relentless sun of some of you down south.
I've had no trouble with the actual strap webbing (except when caused by operator stupidity...). All my trouble has been with the mechanism, whether the actual ratchet/release or the retraction spring. I do keep them out of the elements, i.e. inside vehicle or storage shed, most of the time, but occasionally forget. But keep the suggestions coming, even if they're addressing other points of failure than mine, ones which might apply to others!
Someone mentioned not being familiar with retractable ones. Here's a link to the initial ones I got (This is a Canadian link, in C$, but I expect they're available all over):
General PDP Template Note these are the ones that *don't* last for me.