I have been having problems with my glow plug relay unit ( I think??) basically my glow plugs were first replaced due to white smoke when starting, but once they were changed it then would start but not catch, so my mechanic assumed my glow plug relay unit had blown due to my glow plugs previously not working

so new relay was put in and has lasted around two weeks

.It seems to have blown again. Is there some underlying problem which could cause this as I dont want to have to keep replacing relay.