Parking is a result of the airport design, not the flight plans.
Now flight plans do heavily influence parking - mainly by the number of aircraft put on the airport.
Are you using the Traffic Tool Box SDK Explorer to verify that you aircraft have the correct wing_span= values for the parking spots? That the parking codes match? That the aircraft movement times are when you are watching?
Parking Codes - must match EXACTLY - ACA is different from ACa and Aca
Parking Spot Size - must be as large or larger than the aircraft half wing span value. Be sure you are using Meters to set parking values, not feet. It is very easy to set a feet value and have it round down, resulting in a spot being too small.
They will just sit there and watch the default aircraft come and go.
If the AI aircraft you have setup just sit there, then the time for their departure has not arrived. If AI aircraft are scheduled for departure, and something prevents the aircraft from taxi to the runway - the aircraft will disappear.
But, are these flightplans one airport only plans? i.e. CYHM -> CHYM plans without an intermediate airport?
If I change the time, they will change they're parking spots.
This usually indicates an overflow issue. Various traffic.bgl files are loaded in ascii order of their names.
An old example, but a fellow had a lot of trouble with the SWA B737 aircraft parking in cargo parking at KORF.
After about two weeks, probably 50 posts and a lot of checking back and forth - we found the problem was the size of his NWA parking. He would start is FS at the same time, which would generate an NWA aircraft in the SWA parking spot. Northwest traffic was loaded before Southwest traffic. Then the first thing which happend was the NWA aircraft would pushback and taxi out.
So when he checked his SWA aircraft were at the cargo parking and their spot at the terminal was empty.
At any airport, I always like to test the parking with just the dominant airline. Make sure that airline parks perfectly at various times. Then if I see aircraft out of place, I know it is that that airline causing the problem.