Well. its hard to tell exactly, why they don’t take you, I looked at your works and I think for a junior designer it’s enough. But there can be one more thing that can (i think) help you- create a concept of a very good looking project. It must look like you stole it from the dribble.
I hope you already know that a lot of stuff that we see on dribble is not real. I mean that it’s just nice pictures that will not work in real life, they show best that designer can do in composition or show his new ideas. Soi thinks you need to create something like that and add them to your portfolio, to show your best skills in composition icon design or illustration. And these things easy to show in concepts because you don’t have a client that can destroy all idea, you just create work like you want
P.s sorry for my English:) I’m Russian so it’s hard to me sometimes write in English