It seems a common death of spa covers is that eventually the thing loses strength in its spine along the hinge, and starts drooping until it collapses into the tub. Time for a new cover.
Though it doesn't sound like a hard problem to solve.. just build a triangle truss structure inside each half of the cover, with compression and tension beams..
Does anyone make spa covers like this? It seems like an easy way to get ... a long-lived Trouble-Free Spa™...
If the truss tensioning was done with a cable around the large triangle shape, it'd be possible to re-cinch it tighter over time with tensioning bolts, as things start to sag from age.