Would a second bone graft be possible after one with less than ideal results?

After having my failed tooth implant removed, my doctor did a socket graft. It is now healed but I’m not completely thrilled with the results. However, he told me a second one wouldn’t be possible. He told me to imagine a pot hole and when filled with cement, the defect is no longer there. Therefore any more attempts of pouring bone into the area would not work. Is this true?