If the bag has been stored for a while (especially surplus), all the dirt and dust gets worked into the webbing, and they can be really hard to move. Perhaps go one further than Shewie's excellent suggestion, and try giving the whole bag a really good clean. Again, if its surplus, it might need it anyway.
The best I've found is to unzip any side pockets, etc, and bug the whole lot into a shallow bath with some Oxi Cleaner (or something like it). Soak it for a while to let the Oxi start working. Give it all a good scrub with a vegetable brush, etc (and Vanish type spray stuff is good for getting off serious stains), and then rinse off (with the shower or garden hose), and put it on the line to dry out on a nice sunny day.
Hopefully, the webbing will be fine after that, but someone on this thread:
http://www.scubaboard.com/forums/buoyancy-compensators-bcs-weight-systems/416442-very-stiff -webbing.html suggested 'the easiest way I've found to soften stiff webbing is to loop it over a bar (like a chin up bar) then with as much weight as you can apply on, pull it over the bar in one direction, then the other. Do this as many times as necessary.'
Hopefully, soaking should work, because that sounds a faff!
