After cadaver bone graft with implants is there any reason that there would be bone loss two years later?

My dentist did a three tooth bridge over the two implants upper right in mouth. The bone loss is between the two implants. It started out being sore in the whole area above the bridge. Also had a sinus lift with the implants. Dr says sinuses look fine, wants me to go to root canal specialist and have #6 tooth checked out. It's only two years, I feel it's crazy there's bone loss. It's hard to floss and would that be a factor? Thank you!!