Yuvraj thrives against the white ball because he;s a middle order player and comes mainly when the field is spread in the ODIs. He's got chance to take singles, get set and then in the final overs go berserl. In other words, Yuvi excels when his style of play is limited, ie, in ODIs, it is "singles for the first half hour, then boundaries". And in 20-20, it is "All boundaries". That is why he thrives in such situations. He loves to play in a situation where there is some sort of a target. In test cricket however, the targets are spread over longer periods of time, and requires concentration.
Yuvi seems approach test match cricket in a predetermined fashion, and tries to play sometimes unnecessarily defensively. The problem is, he lacks a good defensive technique, ut has a superb attacking one, And he should utlilize that to his use. So instead of trying to awkardly trying to manufacture a defensive shot to a normal off cutter, he should play it the way he'd play had it been an ODI, "smack it over" the infield. We saw that kind of Yuvi during his glorious 168 against Pakistan last winter. But in Australia, his predetermined defensive attitude cost him dearly. Got out trying to defend deiveries which he'd have gobsmacked had it been in color cricket.