Well..turns out that on the second machine...if I take the same plugin project and relink it, then it works on that second machine, but if I then move that plugin and drop it back on the extensions folder on the first machine. It doesn't work. IT only works on the machine it was compiled with. Both machines have all the same dll's etc.
I've created a new plugin, and on my development machine, if I drop the plugin into the Filemaker Extensions folder, FileMaker picks it up. But if I drop in an extensions folder on another machine FileMake doesn't pick it up. Is there something else I have to do deploy a plugin?