A box drawn from a single-point perspective has a front face that's completely facing the viewer, and a back face that's also completely facing the viewer. The edges connecting those two faces are meant to aligned towards the vanishing point.

None of the edges on your boxes are drawn toward the vanishing point you marked on the horizon line. I can tell that by the ruled lines you drew extending those edges to the horizon -- they hit the horizon, but they're way off from the VP, even going in the opposite direction.

That's also the reason why you're unable to draw the extending lines properly on the rightmost box, because its top edges aren't heading toward the VP on the horizon at all.

Reread the Exercise page, and especially watch the video, because Uncomfortable shows how to do it in the video. Good thing you checked, as you'll have to redo this drawing completely! Hope this helps.