KB,
You do good missions. They've seemed doable when I've joined. I'll join again.
To answer your question, if I'm inflight and not landing soon, I won't join. If I tower sooner than I thought, or rtb is more attractive than getting ganged, I'm in.
If there's a fun furball going on, will I leave it for a mission? That's a reputation/trust thing that comes with time. Because I like your missions, it's a definite maybe, not a no.
I joined ET's and Artlaw's missions because they were well-planned and usually productive. There's a chance any mission will be busted by the enemy. So, build them and they will come.