This problem has a variety of possible sources, and it may be a combination of sources. I can help provide some guidance but you'll need to do some diagnosis on your end as there is not a sure fire answer to this question without it.
I'd like to start by asking some clarifying questions myself - if you can, please comment to provide more detail which can help narrow down the answer. First - does this happen regardless of the outside temperature? Is it easier to start when the weather is warm out versus when it is cold out? Second - how long has this been going on? Is this new, and if so, can you point to something that you did or you noticed that resulted in the car being hard to start? For example, did it have a hard time starting right after you filled up at an unfamiliar diesel station?
Hopefully you'll be able to answer a few of those questions. For now I'll go through what I know of as the most likely culprits for hard starting issues. Remember that it might be more than one of these items combined.
Glow plugs - if your glow plugs are not working as designed your car will be very difficult to start and may run poorly for some time after starting especially if the weather is cold. You did not mention the year of your car - if you have an early 300D (1980 or older) you'll likely have the older style glow plug system that is wired in series. This setup fails very easily - if one glow plug burns out the rest won't work either. You'll need to consider pulling out each glow plug to check for a bad one (it will be obviously if the glow filament is damaged). If you have a later 300D the system is wired in parallel and is more robust (one plug can fail without taking the whole system down). However, there are still a few single points of failure - the glow plug relay could be bad, or the glow plug strip fuse under the cover on the relay. In either case there are some resources in the iFixit W123 section, and elsewhere on the internet, for diagnosing the glow plugs on these cars. Look in to that if you at all suspect they have failed (especially if your glow plug light is not coming on when you move the key in to that position).
Valve adjustment - if it has been over 15,000 miles since your last valve adjustment you will want to do so. They tend to get tighter as you drive, and eventually they get so tight that they can lead to improper compression. This can result in hard starts and poor engine performance. While there isn't yet a guide on iFixit in the W123 section for valve adjustment, if you google "W123 valve adjustment" you will find some good resources.
Fuel leak - a fuel leak can often lead to air replacing fuel in the lines. This can lead to very hard to start engines as the air has to be pushed out through the injectors before fuel can flow properly again. A very common culprit is the old style primer pump. If you still have this style, I'd strongly recommend replacing it. You can find a guide on this on the iFixit W123 section. Also check around to see if you have leaks on any of your rubber fuel hoses and if so replace them.
Filters - if you haven't changed your air or fuel filters in a long time you might consider replacing them. However, this is more often a cause of poor higher speed performance and less likely an issue for starting unless they are very badly clogged or the fuel filters suddenly become totally clogged due to contaminated fuel.
Poor compression - and in this case caused by something besides out of adjustment valves. If your valves are adjusted to spec and your glow plugs are working and fuel flow and air flow are OK with new filters installed then another place to look is a compression test. When in good repair these engines will produce cylinder pressures over 400psi. As they age this naturally drops. However, anything below 300psi is cause for some concern. And anything below 200psi usually means that the engine won't start. Also, large differences between two cylinders (i.e. one has 400psi the one next to it only has 320psi) can mean there are issues with the headgasket or other problems.
Keep in mind that the first two issues are the most likely. Don't get worried prematurely about a compression issue. Go through the steps to diagnose and rule out the first few items before continuing. And don't hesitate to ask on here or via email (address in my profile) if you have follow up questions or concerns.
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