Did my hair transplant failed and can I go for second sitting? (Photos)

Based on the pictures and the history provided, you can undergo a second hair transplant. But you should sit with your doctor and properly plan how many grafts can be taken out and which areas should be properly filled so that you should be satisfied at the end of the procedure.

I'm not quite sure if you can call it failed as I don't know the expectation that you agree with your surgeon. However, you can get another session to thicken the thinning area as long as you have enough donor hair. For further assessment, kindly see board-certified surgeons. 

You can have a second hair transplant if the donor area has a reasonable amount of hair left. Failures are usually the result of technical problems at the time of the surgery. If the first surgery was with FUE, the second surgery should not use FUE or you will have balding in the donor area