I would like a second opinion, why didn't they put grafts in the middle even though I am thin there? Will PRP work? (Photos)

Hello ;

First of all, thank you for your questions and photos.
I don't think there's any error in your hair transplant, because they planned to fill the anterior region for a start. my opinion is that you need a 2nd session. If there are donors in your nape region that can be handed to the second session (I couldn't see the photo of the nape region), you can perform a densification study in the second operation.
Kind Regards

You do have hair follicles present on the top of your head, so PRP could help. PRP with stem cells would be even more helpful but it may not work for everyone. HT is much more reliable and more permanent. I would have waited on the transplant until I saw how you responded to the PRP and then performed the transplant, that way you could have had the top treated as well if needed. It looks like you may need a second procedure to treat the top so that it matches the front. 

First of all, what was the surgery plan initially? What did you speak with your surgeon? Did they deliver something different than discussed? Or was the crown never mentioned?
You still have a lot of native hairs, but they look weak. A penetration to scalp at this stage could have caused a permanent loss on some of your native hairs and you may have not looked different than how you are now. This can be one reason why they didn't want to do work.