For my last three forts, I've had the same problem. I can't export wealth - I get to 2-3k exported wealth on the first few caravans and then the counter never increases no matter how much I trade or give away. I usually get 2-3 caravans and then they stop arriving, same with migrants - I'll get a few waves of them and then nothing. After this no enemies attack either.
Doing some searching I'm guessing I have the "missing caravan" bug. I've tried the 'unitretreival' script from this thread (
http://www.bay12forums.com/smf/index.php?topic=159297.0) and it does find some missing/stuck merchants, which I then try to fix using the dfhack 'tweak fixmigrant' command. This removes them as merchants from the unit list. But even after doing that I still never get another attack, caravan, or migrants even after waiting several years.
This is getting frustrating, does anyone have any advice as to why this keeps happening? Are there certain things that would be more likely to trigger this?