See: 9435, "Total value of exported goods remains zero after exporting goods". More info about your situation and saves showing this behaviour are welcomed.
Thanks, already posted there, but I'll provide more info and upload the save where the export wealth stopped accumulating part way through. This thread was more inquiring if this problem is affecting everyone or just some people, as plenty of people seem to have no problem with nobles and caravans arriving. Maybe it's only caused in v.04 games, and those played/started/genned in previous versions aren't affected?
When new releases come out, people will hold onto old games until the new version stabilizes. Others wait for mods to be updated, and you can't pry the LNP out of even their cold dead hands. Others get overly attached to forts and refuse to give up on their pimped out 50 year running obsidian-casted tower that runs on dwarven minecart computers (even when "compatible", transfering saves between versions is a bad idea). So yeah, a ton of people are playing in older versions.
I've also experienced the problem, flat 0 in exported wealth. I also noticed the curious thing about human caravans.... the first time I thought I just moved the embark tile one tile too far out of range when I was finalizing the position of my fort, but then I got a flood of human visitors. I had ~10 full human citizens by the time I retired the fort 6-7 years later. The next few forts I created were the same way- humans on the neighbor and embark screen, but no exported wealth.
The most depressing part is that on Mantis, this has been confirmed but assigned low-priority. It seems to happen 100% of the time, and completely blocks noble progression. I know, I know, nobles are parasites. However, they are status symbols. When I embark and create a new fort, I want a bloody barony at least out of it to tell the rest of the mountainhome this place has its shit together! Hopefully it'll be fixed soon...