Hi guys!

Have a Chevrolet Cruze 2.0D (Z20D1), around 40k miles, having some cold-start issues.
When trying to start the engine, the electromotor seems to work, but usually the car starts after 3-4 seconds, and sometimes does not start at all.
Few months ago, took the car to a service, and the diagnostic was: glow plug relays missfunction. Component was changed, no difference observed.

This weekend, our Chevy decided not to take us home from mountains. It was not started for 2 days.
Usually we use the car every day.
Probably I've forced the battery to much, trying to start the engine. Finally we've stared it by connecting to another car's battery.
The next morning, the car started, as it started usually in last months, first try, but after several seconds.
The glow plug light goes off before starting the engine...
The battery is still in warranty and it's good: was tested with special equipment.

What do you guys think could be the issue?

Thinking might be the glow plugs. Wanted to measure the resistance of glow plugs this morning, but had issues removing the connectors to the glow plugs.
Do I need to pull harder, or there is some hidden trick for removing those connectors?

Thanks,
Adam