BMW E90 320d: Starting Problems After Long Idle

I have a 320d that has difficulty starting after being idle for a few hours. The longer it sits, the worse the problem gets.
The battery and glow plugs are fine.
I've also checked the oil feed lines, and when I removed the intake manifold, I replaced all the lines underneath it. I also replaced the O-ring on the metering valve at the high-pressure pump, as it was slightly damp all around.
I also installed a check valve after the filter, as it was the only place where there were no plastic pipes.

Hello,
Does electricity actually flow through the glow plugs? Can this be measured with a clamp meter? (Or observed through the openings in the spark plug well for the electrostatic discharge electrodes).
I once had a problem with the plugs.
Otherwise, it's heading in the direction of (BMW) diagnostics.
hg
Herbert.

I haven't measured the power consumption, but it should be detectable through the diagnostic system. There's nothing related to this issue stored in the error memory.

Edit: We're currently around 8°C, so it should start right up without needing to be pre-heated.