On my 70s era bike, a couple of times now all of sudden my rear derailleur stopped shifting down to lowest cogs for no apparent reason -- at least none related to cable tension. Both times it turned out the pivot point of my old derailleur had gotten gummed up so wouldn't allow the derailluer to release all the way outward even with cable entirely slack and limit screws set correctly. Take the wheel off the shoot a few drops of lube into the pivot points while you move the derailluer inward and outward with your hand see if that frees things up.