I have found when shooting spots I tend to try to MAKE it happen, as opposed to LETTING it happen. Somehow the fluidity of drawing and releasing smoothy in a syncronized, rythmic fashion morphs into something more rigid, measured, analysed etc. etc. and as a consquence my instinctive or natural abilites are overridden or tainted by this rigidity. This is just another way of saying what others have said about tensing up. Somehow we start to distrust our instincive abilities -- really all we have to do is to BELIEVE, and it will HAPPEN, regardless of the target.