Shallow traps on bath/shower shouldnt be used if the waste goes to a stack, ideally need the 76mm seal traps, to lessen the risk of the trap getting pulled. Just a theory, but if the Macerator discharge is gunging up the opposing connection for the bathroom, and restricting the diameter of the pipe, then its possible shallow traps might be getting pulled on bath/shower when a discharge is made from another appliance on that run. Seen it happen with a Waste disposal unit on a kitchen sink, connected opposite a basin from the downstairs WC, the basin was repeatedly blocking.
Capped off the basin connection into the stack, put in a new waste run to an external gully and no more issues.
Capped off the basin connection into the stack, put in a new waste run to an external gully and no more issues.
