Ah, OK.

Yeah, time to open it up and take a look. I guess I would open up both (the one that works and the one that doesn't) and see how they differ. Possibilities:
- ratchet wheel is upside-down
- washers were put in in the wrong order, or udside-down
- the spring-metal tab that engages the gnurlings on the ratchet wheel might be bent or broken

I guess there are other possibilities,,,

Here is an exploded view. This is a downtube set, but the ratchet mechanism is the same:
http://www.velo-pages.com/main.php?g2_itemId=12604
Actually, looking at it, parts #4 through #16 are the same as the thumb version (aside possibly from the length of the lever itself).

Part #10 is the ratchet wheel; #5 is the tab that engages the ratchet gnurlings on the ratchet wheel.

Part #16, the screw that holds it all together, on yours has a d-ring. Some have no d-ring, some have a plastic wing nut. No real functional diff.