These things aren't designed to seal liquids. Period. Anythign with a lot of liquid in it isn't going to seal well.
I swear I've read to freeze prior to sealing things with liquid in them, and things like berries or bread in the book that came with mine. Here is a link to the manual and an FAQ that discusses these suggestions.
Proudly Smoking on a UDS, PBC &
Accidental Winner of the 2012 "Saucy" Throwdown.