Plastering has always been dog eat dog, don't know why, and you don't get it with other trades really. I remember site bashing nearly 20 years ago, T***s claiming good plots by fixing the angle beads and sabotaging your hose pipes or nicking your 110 leads.
Has to be to a point were your earning good money from spreading, otherwise you may aswell do something else that's less toll on your body. I've a pal whose a window cleaner and he makes 800-1000 a week steady away chatting to housewifes, brews and biscuits at the ready, never breaks a sweat.