It's been 12 months since my hair transplant. I still have empty spots and the hair is still thin! Is it OK or not? (Photo)

 I think it seems to be low density and un successfull result for the 12 month post op.. you may need a revision surgery to get more density .The most outstanding reason for poor growth and ineffectual  hair restoration results is insufficient number of viable graft placement as a result of intra operative  damage of follicular units ( during insertion or high transection rates during harvesting, long time of grafts out of body, dehydratation, poor FU storage conditions like high room temperature and storage fluids etc.)

At 12 months past your transplant it's unlikely you will see any further changes to your hair. Since you're still unhappy you should talk to your doctor, it is possible that there was a miscommunication in what your results would look like. In general transplanted areas will not be the same density as normal hair, and it's hard to say whether your transplant was a success without seeing before pictures. 
Many patients get a second transplant simply to fill out and add further density to the recipient area, this may be your best option to get results you are satisfied with.