Best Of

Being brutally honest? Your fundamentals (such as anatomy and lighting) are either novice level or non-existent and there's nothing about your work that feels concept art besides having a turnaround in it. Its not a surprise why you haven't gotten anything back as most employers are used to hiring guys with top notch quality and don't like to settle for less.

You should find actual courses online that explain what concept art is and find general art courses that teach you about the fundamentals. Check out Schoolism with Bobby Chiu. He has a number of artists who teaches courses on these topics and do a good job of it. I can attest to this because I've bought his courses before.

https://schoolism.com/

There is another one I've seen that looks good called Visign, though I haven't purchased anything from it so I have no idea if the instruction quality is good. Still, it looks good and might be worth a look.

https://visign.com/courses

But yeah if I were you, I'd not worry about applying for jobs. Instead just focus on improving to get much better and when your work starts to look like what currently working concept artists are doing, then apply.