2006 Ford Focus Hello Car Diesel Having Trouble Turn Engine On This

Hello, my car (diesel) is having trouble to turn the engine on. This is happening randomly. Maybe once a week, for 3 or 4 months now. When I turn key, engine doesn't started. It's like a car has no gas. It make all that "right" sound and show "right" lights, but engine simply doesn't start. I repeat that for 3 or 4 times, and than engine turn on with no problem. The car is Ford Focus, 2006, 2 TDCi. Any ideas or suggestions?

Best regards, Grega