I have an airport which I built, and set all the AI traffic to come from a certain series of aircraft I had created.

One of these aircraft is very large, and it just never seemed to appear in the sim.  I just assumed that it was too big for FS-2004 to handle as AI and that's fine.

Now I've been building all this up over many months, probably since about last summer, and this aircraft wouldn't show, so I carried on.

Today I was flying and I saw something in the sky coming toward me, and I thought "What could that be?"

Lo and behold, here was the aircraft that I thought didn't work.  I went something like eight months without seeing it, and all of a sudden...

Does anyone have any idea about what might cause an aircraft to behave in such a manner?

size of parking sounds like the problem. Mike Stone's C-5 has the same issue. The default parking radius is still to small.

You can edit it I believe in AFCAD and make the radius smaller of the missing airplane

Bill, are those afcad parking spots named cargo and not mil_Cargo?

From this thread here everyone has posted the afcad mil_Cargo needs to be adjusted

http://www.flightsimmer.com/forums/showthread.php?t=32228&highlight=Stone

from Michael MacIntyre
In order to use any of the three C-5 models (RL, KM or MS) as AI you must adjust the parking radius to 44 meters or less when you have Ramp Mil Cargo parking spots set at their default radius in your AFCAD airport bgls. This aircraft parking radius adjustment is most easily done with the aircraft editor in Lee Swordy's AFCAD2 program.