Are these default AI? or have you added AI?
If you have added additioinal AI (or even dafault AI I suppose) and they land but have no place to park they will dissapear.
From the Traffic Tools Readme.htm
Aircraft disappear shortly after landing
This usually means there are no gates or parking spaces for the aircraft. This can either be because there is no parking of the appropriate size at that airport, or all the appropriate parking spaces are already filled. Jets the size of a 737 or larger require a gate (gates are all larger than ramp parking), so if a 737 lands at an airport without any gates then It will disappear. Also see the next paragraph.
Can I add or change parking spaces?
You can use a program called AFCAD to add or enlarge parking spots
Cheers,
Dave