I'm not sure what you mean by model converter unless this is an FS2000 aircraft. In this case the only converter you need is the MDL Converter tool posted on the official M$ CFS2 site.

The problem is most likely in the DP file. Open it in Notepad & check the entered_service= date under the [MISC_DATA] header. This must be earlier than the date of the first mission in the campaign.

If the aircraft has no DP file it will need one. If the [MISC_DATA] entries are missing paste them in from one of the defaults. This is the entry from the default Wildcat.

[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

Your problem is most likely with the DP file itself. The MDL file only needs converting once. If the external payloads display & fire it's already done. Try deleting the CDP file. CFS2 will write another one when you use the aircraft.