The most consistent problem that I'm noticing is that sometimes the lines in some axis of the box intersect even though it's inconsistent with the suggested vanishing point of the perspective. One thing that I like doing it's drawing the extremes angles in the outer part of the object and then put lines on the middles with "in-between" angles that are just an average of the two extreme angles. it works when doing thumbnails too!

Learn how to construct and use your vanishing points: there are no secrets.

One thing that may help is to make it more clear which sides of the box are facing towards you, like by perhaps making the lines facing us thicker. Overall it looks like your boxes are looking better the more you do. You want to be careful that all your perspective lines are converging towards at least about the same direction. For instance, in box 13 they seem to be going different directions. But, again, it looks like you're getting better with that as you go along. Also, remember that on a rectangular prism all four lines/sides that are parallel go towards the same vanishing point, not just the two that are on the same face.