What are some signs that the bone graft/sinus lift has failed?

A few things: You have have an infection at the site, the gums may start to open, fluid may be draining from the site, there can be congestion of the sinus cavity, the bone graft on the x-ray does not look dense. If this is a concern, see your doctor asap.

Prolonged sinusitis: Prolonged sinusitis may be the sign of failed sinus lift procedure. However, if the procedure is done well, in many case, increasing bone height will still be found at the end of six months. Sinusitis can be controlled with a single dose of levaquin, (levofloxacin) 10 days. Regardless of possible complications, sinus lift is a very predictable procedure and most often will succeed.

Pain, Pus, Swelling: When a sinus bone graft fails, there is often pain associated with this. Swelling will occur, and an x-ray or cone beam image may show disorganization of the graft. Sometimes, the incision line is opened and exudate may come from the site. Sinus congestion may also occur. Antibiotics can often prevent failure of the graft if infection has occurred, however, if not, the graft needs removed.

Graft loss ; infect: A failed sinus lift, may present as chronic sinusitis, an infection at the surgical site, a loss of bone density, a migration of bone position. It may also present as boney particulate coming into mouth or out the nose and or oral fluids from mouth coming out nose. Additionally, a premature dental implant placement into an immature graft site may cause graft dislodgment and procedure failure.