I've done multiple full renders with no errors in the server log

but I'm not sure whats wrong

Are you running the command from the console I.E. dynmap fullrender world or are you running /dynmap fullrender in game?

If you run it from the console you need to specify also dynamp fullrender world_nether and dynmap fullrender world_the_end.

If you aren't seeing it after that make sure the worlds are enabled in world.txt and you can see the icons for those maps in the sidebar