Is the exclude parameter the only one you’re using? If you’re using others too, be sure to separate them using ampersands. The exclude parameter accepts IDs only, comma-separated if you need more.
I see a couple of forum threads about ‘exclude’ not working, but most of those are really old. Basically, “exclude=12,14” should work, assuming 12 and 14 are valide tag IDs (not the IDs of the post they’re related to, for instance).