There are many possibilities & I would always recommend a check for codes on a diagnostic tool such as Mercedes STAR to see if any codes are stored.
We see a few issues - check the low pressure lines to the fuel filter for condition or leaks - even a very slight leak can cause air to be drawn in causing starting problems. If in doubt then replace the hoses & use the correct clamps on the ends of the hoses.
I would also replace the fuel filter with a genuine filter as these can cause problems.
Crankshaft sensors can cause starting problems - usually more from hot but I have known them cause problems from cold - this will usually store a fault code in the CDI control unit.
Also ensure the engine software is up to date - I have known issues like this with older software versions & Mercedes have improved this with later versions. STAR diagnostic tool is required for this.
Also if the glow plugs short out it can often blow the glow plug control module on the front of the engine - so if the glow plugs are not being actuated then this may also be the cause.
Best regards, Paul