I see the same problem here that I see in tons of iso work. It's like, you made a big box, then started adding more boxes to it for details, and it's obvious in the finished product. It's very difficult, or impossible to imagine all the details on some object you have not interacted with intimately. This is why people always say to look at references. A barn is not really just a big box, a house isn't either. There is a logic to it's construction that is apparent in how the finished thing looks. I know you're not making a barn, but look at this as a structure:
See, it is a very complicated object, not just a big square. So basically what I'm saying is, if I asked you to draw a chair, you could make a flat square with 4 square legs coming down, and a flat square at 90 degrees from the seat, but thats boring, while chairs can be interesting and vastly more complex then that: