+1 on the burping or priming of the cartridges; that's pretty much critical with the black berkey filters. Also, you can expect to have an inch or a little less of water remaining pretty much constantly in the bottom. As others have pointed out, just not enough pressure with that small remaining amount, to overcome the resistance of running thru the cartridges. Having more water in there not only increases the positive pressure on the cartridges, it also increases the surface area of the water in relation to the cartridges, which also helps to speed up flow as well.
If you haven't primed them, you probably want to remove & clean them and then re-install them. (Obviously, be sure to use clean, potable water for both cleaning and priming.)
Priming PDF: http://urbansurvivalsite.com/files/P...key_Filter.pdf