Pwedeng:
1. Mahina na battery mo. Have it checked.
2. Baka sira na glow plugs mo. Umiilaw ba glow plug sign once you set the ignition to on?
3. Problem with the starter motor.
4. Baradong fuel injector
5. It's normal in very cold weather for hard starts to occur. When the engine heats up, ok na.

Have it checked with your suking mekaniko. Goodluck. :D