As others have said you didn't put on enough N unless you had a lot of residual N in the soil. If i recall correctly it takes about 2.2 -2.5 lbs N per bu. Another thing in your case could be your climate, spring wheat likes it on the cool side when it produces tillers, and during grain fill, it is possible that you are too warm for that to happen. Did the plants have any tillers and if so how many with heads on them?