Tapering i have a delta 6" joiner and i'm having a problem with tapering. as i run a board through it starts cutting just fine ,but about half way through it stops cutting and leaves me with a tapered board. i've checked for a tilted table with a long straight edge and both tables appear to in alighnment any help?

First, unplug the jointer, then, insure that all the cutters are set to the same height. Raise the outfeed table with a straight piece of wood or a framing square sitting on it and extending over the cutters. Each cutter should just touch the board without raising it. The infeed table is lowered by how deep you want to cut.
Always look at the grain of the board you are cutting. Never allow the cutters to strike grain that is running down into the cutters. It will chip out chunks of wood. The grain should be tapering down away from the cutters, back toward you, never toward them.

The tapering you mentioned usually happens when the outfeed table is lower than the cutter tops.