Oh, I definately understand what you mean about drawings not looking like their intended brand... I cannot draw or come up with a drawing of a concept of a BMW for the life of me. There is just some sort of proportioning that I don't understand that makes any attempt at one by me just not one.
Was it you who mentioned that they only used pen for their drawings? If so, that is your #1 issue right there. Your design is limited to your very first lines that are put on paper. If you feel that some element in the design could look better if tweaked just so, you are unable to change it without having to begin all over again. That also makes it very difficult to experiment with different shapes. For example, with a lot of your drawings, it is evident that you draw what you know. Your hand knows what motion to take to give you a certain shape, and therefore you follow it knowing that you will get your cleanest line that way, and will not have to go back and re-draw it. I think that is also the reason for so many variants of what seems to be the same vehicle. All of this experimenting that could have easily be done in minutes on a single drawing, all while erasing the designs that you feel are not what you were after or had originally envisioned.
Definately take your time with your drawings, don't be afraid to go back, erase, re-draw, and even erase again. I'll tell you what, most of my drawings take me hours, yes hours, to do. First several minutes I may get a desired shape down, or an idea of a shape, and I work from there, but most of the time, 90% of the lines that I have on a finished drawing were not there in the beginning, or had been erased and redrawn ever so slightly. The Oldsmobile that I had done for example, was started months ago. The other day, I probably spent 5 or so hours tweaking and finishing it. I had the general shape down, an idea of what I had wanted to do, but nothing near what I got out of it.
Any way, I do hope some of my suggestions help, I am in no way ragging on you or your drawing, I just want to help you get to the point to where you don't hear the same old comments from everybody all the time! (plus, I would have killed for critiquing like this when I was getting started drawing!)