After reasonably (!) successful and reliable sourdoughs, I had in succession one very flat loaf and then a cowpat (overproofed) - the cowpat was a new recipe but the flat loaf was tried and tested. Even the cowpat seemed relatively fine - not very active bulk fermentation, so fridge overnight, it felt heavy when I shaped, then fell apart before final shaping... The thing I'd changed - was to a new flour!
The new flour came through Bakery Bits - a 100% stoneground white, which I thought would be great. Looking at the label, it comes from Little Salkeld: