What are the signs and symptoms of the bone graft failing?

It not a necessity to have a bone graft for every implant, every case is subjective to the patient.
Why did the implant fail is the real question?
Implants are still vulnerable to infections, i.e, periodontal infection. Did you have sufficient bone to begin with? Meaning, at the time of initial placement, was there enough width and length to place the implants? Was there excessive force being applied to the surfaces of the tooth? All this can affect the longevity of any dental implant.