Have you checked that the glow plugs are actually working? Should be about 11 volts on the centre terminals while in operation. Try pumping the primer before starting, if that improves things you have an air leak in the fuel system. If all that fails, sounds like the compression is low, leave the ignition switch in glow plug mode until you hear the relay drop out, probably about 20 seconds before trying to start it, repeat again if necessary.