I had this problem for the first time last month. I had a 5 gallon corny with bright beer (so no hops or trub to clog the dip tube), but it wouldn't dispense but a trickle of beer. Gas side was fine: full tank with gauges showing correct readings, no leaks, pressure relief valve was vented a few times to confirm gas going in and maintaining pressure. Had to be a block in the dip tube or the fluid out post since I knew the same cobra tap and QD were working fine on another keg. It turned out that the dip tube formed such a snug fit inside the post that it rotated during tightening of the fluid out post with the cresent wrench--resulting in the dip tube opening being flush against the metal bottom of the corny keg (and acting like a valve whenever the tap is opened). The solution was to gently "loosen" the fluid out post just enough to move the dip tube so that fluid could flow again, but no so much that CO2 would escape.