If you've hit a plateau w glute gains, it's often due to muscle imbalances. #1: Are you hitting isolated hamstrings? #2: Is your core/back/upper-body strength good enough to allow you to hit those good glute work-outs properly?
Are you lifting heavy enough? Are you lifting TOO heavy and not using proper form to target glutes optimally?
Are you eating enough protein?
Are you sleeping enough?
Are you targeting Maximus, medius, and minimus?