Maxillary sinus augmentation with a lateral approach (MSA) is a well-established treatment. In this prospective study, we evaluated risk factors for postoperative bone-graft displacement and report the clinical application of long-term resorbable GMEM-B-2 (L-lactic acid/-caprolactone [P(LA/CL)]) as a barrier membrane to cover the open window in the lateral wall in MSA. Twenty-four patients underwent MSA according to the relevant criteria; CT data obtained before and 1 week (1W) and 5–6 months (5M) post-MSA, bone height changes, bone height reduction rates at 1W and 5M post-MSA, bone-graft displacement measurements, and risk factors were examined. All patients showed bone height increments (p < 0.005). However, no difference was observed between 1W and 5M post-MSA. Bone-graft displacement was observed in eight patients; the reduction rate from 1W to 5M post-MSA was 8.38% ± 4.88%. Sex, septa, maxillary sinus floor–palatal bone distance, and maxillary sinus floor–natural mouth distance were associated with bone-graft displacement (p < 0.05). Height from the maxillary sinus floor to the palatal bone and the sinus angle influenced the augmentation amount (p < 0.05). The L-lactic acid/ε-caprolactone [P(LA/CL)] membrane compared favourably with other membranes and may be useful as a barrier membrane for the MSA open window.