Thanks Folks,
It's a 4wt Pieroway, not sure of the action, with a 5wt Scientific Angler line, not sure which type anymore.
Rod and line recently cleaned.
It's not looping around the rod tip, that would cause a sudden change. What's happening is that it's normal at first and then gradually gets worse and worse.
Fatigue is a thought, however, I'm able to do the same short casting stroke at the beginning with a sharp stop, giving a tight loop, but where you would expect the loop to unfurl and shoot some line, at a certain length it just loads the rod but won't let more line through. Like a sticky spaghetti noodle.
What I'm not hearing is 'oh, that's just this problem', so maybe the line has reached its past due date.
Trying on grass for a while is a good idea though, for one, I need more practice, for two it'll tell me if the water is getting into the line somehow...