Debriefings aren't that hard to understand. The game looks at the every single stage. And it plays ALL of them that evaluate to true. A lot of beginners make the mistake of thinking that it only plays the first true one it finds or they don't quite understand which ones are going to evaluate to true but it's quite simple once you have the hang of it.
Suppose you have a mission where you have 2 enemy ships and you win if you kill either and get a bonus if you kill both. Many people would do this.
Stage 1 :
when
-is-destroyed-delay
-Big Bad Enemy Ship
-0
You Win!
Stage 2 :
when
-is-destroyed-delay
-Not So Big Not So Bad Enemy Ship
-0
You Win! (a bit)
Stage 3 :
when
-is-destroyed-delay
-Big Bad Enemy Ship
-Not So Big Not So Bad Enemy Ship
-0
You killed both ships. You double Win!
However this is not the correct way to do things. If you kill both ships you'll get all three messages. If you changes the messages to
Stage 1 :
You killed the Baddzors!
Stage 2 :
You killed the somewhat Badzors!
Stage 3 :
You cool!
Then it works. The skill lies not just in making the trigger but also in realising how often that trigger will play. In the above case the mistake was not realising that all 3 could play at the same time.
From the sounds of it you've added an AWOL debrief stage (Good! I hate it when people forget those!) but not realised that there are other occasions it will play cause the trigger isn't correct.
