refuse to replace goods still under guarantee

Purchased a defy dishwasher in October of 2014 with 2 year guarantee.have been expirencing problem with the machine since November.despite numerous calls to defy service centre in the last 6 months ,a defy technition has come only twice.and nothing has been done to fix my unit.even my calls to defy to have the machine replaced have been thwarted by a response of \we don't replace until we have cheched the unit by our technician."Its been 6 long months of calls and hassles from defy service centre in durban.kzn.please help."""