I've searched but not found anything quite like my issue. I have an '85 Euro TD NA 5 speed that is having trouble starting when warm. Compression is good, injectors are new/rebuilt (Monarch) installed less than 1000 miles ago. (Thanks, Greazzer!)

Symptoms are as follows: Car starts well when cold, first time. When warm, car occasionally would crank/no start. Letting it sit a couple of minutes seemed to remedy the problem. Now it happens on every warm start. I can crank it (long) twice with the throttle wide open, and eventually it will start on the third long crank. Second crank it will want to start but can't quite.

Definitely seems like a fuel issue, so I cracked the injectors to see if I was getting fuel when warm (yes). It's possible it's a clogged fuel strainer, but that would also affect cold starts. I am planning on pulling the strainer and cleaning it when my tank is empty, as well as running a batch of biocide (last down 10 years ago, and no veggie fuel since that I know of).

Glow plugs are new, as is the GP relay.

Any ideas where to look?

Sounds like air ingress in the fuel system. If the compression is good as you say, the glow plugs are totally unnecessary for any start within 2-3 hours of a cold start. As long as the engine still retains some heat, it should fire up easily without the need to glow first.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Ck for oil in shut off pot and lines?

I believe you may be having a vacuum issue. I had a similar problem with my 84 euro 240d a few years back. I'm assuming since your car is a euro model that it has the euro m pump with the vacuum shutoff valve on top of the injection pump. There should be a vacuum line going to the shutoff valve. Next time the car is warm and doesn't start, pop the hood and pull off the vacuum line from the shutoff valve. Then go try to start the car. If it starts, then you probably have an issue with the vacuum element on your ignition switch.

In the past people used to say if the valve adjustment was too tight when it got hot it could manifest itself and make it harder to start. When they get hot the valve stems elongate making the clearances even tighter. (In the Mercedes Manual you are supposed to take the compression with the engine hot.)
When drive the Car and up to temp and you stop and turn it off for a while afterwards the cylinder head temp spikes. Little coolant circulation and no oil flow to take heat away.

Checking the valve clearances is not hard. Adjusting the valves a little harder.

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84 300D, 82 Volvo 244Gl Diesel

Well I've pulled the fuel strainer, which looked pristine, so that's not the issue. PO installed oil traps in the vacuum lines, I'll check they are clear. I adjusted the valves about 1000 miles ago, will check that again if all else fails. Going to check the vacuum per KJenkins recommendation next. If that's not it, I'll try to figure out where air might be leaking. Any likely locations?

Well, it was the vacuum switch on top of the euro IP. I'll have to track down the leak on the ignition switch. Thanks, @KJenkins!

A vacuum "leak" won't cause the stop lever to stay down. Vacuum is applied to hold it down. What CAN happen is that the lines are reversed on the vacuum switch on the ignition lock and would cause vacuum to be held on the IP diaphragm.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Be careful when diagnosing the cause. If the ignition vacuum component where bad. Not only should it be hard to start hot. It probably should shut down after a cold start. As the vacuum builds.

If the engine has too much blowby can also shut down a warm engine trying to start. Whatever is going on can be proved just by leaviing the vacuum line off the shutoff. Just before trying to start the engine hot. Engine starts easily it is a vacuum issue. Engine does not start it may be a blowby issue.

Engine crankcase pressure can be felt by the start/stop control device on the injection pump. Male sure the vent hose for the engine is clear. Off the top of the valve cover.

Just thought I would mention this not that it is the problem. Just a remote possibility. If the engine blows off the oil filter cap when loose is another test for too much blow by pressure. Cap can dance around but should not be blown off. This is a test for a hot engine by the way.

These injection pumps are radically different than the Roosamaster pumps in my trucks, so this may not apply at all:

When I get this problem on my trucks, it's caused by a worn injection pump which can't deliver enough fuel to start when hot, but delivers plenty of fuel to run even when hot. I trickle a quart of water over the hot worn pump to cool it down for a hot start.

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'75 John Deere 450C (times 2)
'83 240D plus parts car
'87 F350, 6.9L IDI
'89 F350 crewcab, 7.3L IDI
'89 F-Superduty, 7.3L IDI
'91 F-250, 7.3L IDI
'91 F-350, 7.3L IDI
'94 F-350 Crewcab, 7.3L IDI
'94 E-350 Ambulance, 7.3L IDI

Thanks for the suggestions. I've rearranged the vacuum hoses (incorrectly) in the engine bay and now it won't turn off, but starts up great! I need to sort my vacuum hoses, I guess.