Tactical flaws are the main reason for people failing to score/win games. That makes it a good place to start.
To a lesser extent, it could be motivational. A striker missing 2 sitters will have less confidence when the 3rd one comes around. Ask Torres what not scoring does to your confidence.
More than likely though it is tactical, but let me ask you this, are you having problems with all your strikers or one in particular? Personally, I have a problem with one striker only. He's young and inconsistent and he misses sitters almost every match.. I find that I have to play him in a different role and I constantly have to take pressure off him. I am also in the process of teaching him the Places Shots PPM so hopefully that helps. All my other front players do well though and they're scoring at a rate better than the best strikers IRL.