Having starting problems of my kia ceed 1.6 crdi model 2010

if someone can help me i would be very glad. I appreciate any helpattempt.

The car i am using has difficulties sometimes when starting the engine, independent of the engine temperature sometimes it doesnt start with the first crank attempt. I also had noises from the fuel inejectors which dissapeared after maintaining the fuel injectors but the difficulties when starting especially when the wheather is cold, remained.

I also changed the glowplugs. It seems better now but i think the car should start faster.

Something i noticed is when i turn the ignition key to the on position and keep it there for a long period of time (30 seconds and above) it never starts.

Is this normal?
When the glowplugs heat up in the first couple of seconds when turning key to the on position, do they cool off again after the glowplug sign vanishes from the dashboard??? İs that the reason why my engine wont start after waiting too long. Is that normal?

Thanks for any help