Chamakh definitely has a confidence and motivation or mental exhaustion problem, and he has also supposedly been disrupted by off-field stuff.
However when you think back to his original contribution - something like 5 goals in 10 appearances when he first showed up - we were all saying 'Chamakh is great, he gets them in whatever it takes and we need a player like that'. Winning scrappy penalties, bundling the ball over the line, etc.
In retrospect, it's perhaps more the case that he doesn't have the instincts and technique to convert chances naturally.
When he was playing well for Bordeaux, his vigour and attack at the ball were exceptional. This is a guy who was at one stage meeting headers at full speed with half of his body above his opponent's shoulder.