First thing is that you didn't do the extending of the back lines. As is stated on the challenge page, the back "Y" is the hardest one to get right, and, by not tracing that after you did the boxes, you probably didn't notice how well you're drawing it. Pay attention to your back lines.

Furthermore, the hatching on the front face of the cubes is not really what it should be. You need to make the hatched lines go from an edge all the way to the other. You should also ghost those lines to get them very straight and evenly spaced. This will make your boxes feel more solid.

Also you got a lot of empty space on you page. It's a good thing to extend the lines all the way till off the page or when it gets too messy with other boxes. It can be a great visual queue to check if everything is really lined up.