Why don't some add-on A/C work as AI in FS9?

I've noticed that a lot of third party A/C (ones that fly
and display beautifully in external views) will not function
as AI traffic. Is this a common problem? With a simple fix?

I have added them all into my A/C database with TTools, and have
been very careful to do all that properly, and have added plenty of
examples of all types of parking (with Afcad) at my test airports.

Anybody had the same problem, and/or have a clue as to a solution?
Thanks.
WS