Hi GNDD. It's been a while but I think this should fix it. Open the Corsair DP file in Notepad & check the entered_service= entry under the [MISC_DATA] section. Make sure the date is earlier than the date of the first mission in the campaign you wish to use the aircraft in. If the [MISC_DATA] section is missing you can copy it from a default DP. This is from the default F4F4_WILDCAT which can be used in all default missions.

[MISC_DATA]
unit_family=1
category=1
allegiance=3
max_group_size=8
min_speed=240
cruise_speed=405
max_speed=607
min_alt=50
cruise_alt=5893
max_alt=8839
entered_service=11/1/40
crew=1

Remember to delete the corresponding CDP file each time you edit a DP.