Bone loss shortly after implants placement?

Authors
Extraction of 15, 16 was done in August 2020 with bone graft. In January 2021 14, 15, 19 implants were placed. 14, 15 were uncovered in June 2021, 19 was not covered by the gum. All implants were restored in September, October 2021. There is a significant bone loss compare between June and October X-rays. There is a 5mm pocket and bleeding on probing. What could be a reason of the bone loss? Can it be fixed? Thank you.