I notice as my chain needs adjusting and I move the wheel back, the angle of the rear brake plate lever on the wheel, changes and the brake rod fouls against the nut on the frame stud, that holds on the mudguard stays. Only by shortening the chain can I move the brake plate lever higher and the rod doesn't hit the nut. Could this be affecting your braking?

You may have contaminated brake shoes
Clean with petrol and a wire brush and leave to dry.
My M21 rear brake is pretty rubbish as grease has got to it.