I have had this problem for a while now.
A little history preface:
The airport of Benevento was a military one in the mountains of south Italy. In the fifties was decommissioned, and fell in disuse, lately though it's being used by ultralights.
In my sim, I have, thanks AFCAD not only rebuilt, but given to my virtual Benevento airport the fictional ICAO code of LIRB.
Now, I've used it for a while with no problems, landing with my planes. The problems began when I decided that use a little AI traffic in it.
Little in the sense that I thought that of a pair of Cessna going between LIRN, LIRI and my LIRB would be nice.
No matter what I do, though, any time I check and re-check things to see WHY, there's never any traffic on my custom LIRB airport.
I have pretty much given up, but before covering this little project of mine with a tombstone I've decided to ask on the forum.
The airport has been defined perfectly and works correctly, with the right parking spots (I can use AFCAD well enough not to fall in this kind of trap) and the traffic I attempted to use on it was defined well too (and repeatedly changed and tweaked in the hope it would work).
Is there a reason because a custom airport cannot be used by AI?
If you know, please let me know as well. I'm about to go bonkers.