Ever notice how those slumps, always seem to follow some of one's best shooting periods?
I think, at least for me, its because we get a little lacking in attention, and concentration....because we were hitting well and let our guard down so to speak.
In those rough patches, make sure your shoulders are square with the target, and keep your release hand very close to the face upon releasing. Another thing to watch for, is putting pressure on the arrow. Draw as normal, and look and see if you are bending the shaft while in the hold position. If so, find where the pressure is coming from by trying various movements until you see the shaft come straight. Then work on eliminating the problem..