my diesel car is hard to start in the morning but it starts ok for the rest of the day why?

no matter what the air temp is my car is hard to start but once its been used it will start fine for the rest of the day
i have changed the oil air and fuel filters and replaced the glow plugs i have also replaced pipe from the pump to the injector pump
but no joy the problem still there and i am stumped as to what it could be