I've noticed that most of my downloaded addon aircraft do not show up in the dropdown box when selecting an aircraft for campaign missions. Any idea of what causes this, and is there a fix?

The strange thing is that a few do... the stock Flight 1 Pearl Harbor P40 (from Flight 1's "Pearl harbor" addon), as well as a few (but not all) of the CFS2 AI aircraft that I have converted to be player flyable in the game.

What gives?

P.S. I've Googled this problem/question for quite some time, but have only stumbled upon 1 or 2 people asking about this, but with no answers.