Against Chelsea, Walcott had about 6 shots on goal. 3 clear chances, only one which was saved by Cech, the rest of them were "how in blazes did he miss that" opportunities. This has become a pattern.
Walcott gets caught between responsibilities of goal scoring and supplying. As a winger he is supposed to do the latter but with injuries to Eduardo, Adebayor and RVP and a drought in goalscoring, he has had to step up as a second striker under Wenger's shuffling of positions.
His shots are tantalizingly close to being considered crosses but they have too much power and are not carefully timed for other players to take full advantage. It also appears that he is taking shots at goal but the angle is wrong and has no bearing to where the posts or the goalkeeper is situated. In the end it really is neither one nor the other. The great tragedy is that with his pace and on ball skills he is always going to give you chances. He just has to make better decisions.