my car ford ikon 1.8d is having starting problem.its starting only after 2 to 3 minutes of continous cranking.

its a diesel 1.8d ford ikon.heating plugs and battery has been checked by service person and found good.its taking long cranking time for starting.once the engine is started in a day,its working fine and starting within 5 seconds.but if the car is stopped for more than 6 hours,its again taking long time to start.