macerator intermittent fault on shower

[steve6266]

fitted 3 way macerator in a student apartment. all working well until student goes home weekends then when returns on monday the shower waste backs up as the macerator fails to kick in. (basin and toilet no issue).
once reset it works fine all week as its used daily then once idle for couple of days same problem. changed the unit and same problem.....once reset run shower and motor kicks in works perfect

 

[rpm]

Is someone else using the shower whilst the student is away?
Is it still the original non-return valve in which case it could be slowly letting water run back over the weekend, doesn`t explain why the unit doesn`t kick in though unless the shower tray is to low. Which brand & model of macerator is it and does the discharge pipe run vertical or horizontal from the unit and how are you resetting the unit?