The Retro should result in an Actionable Item and follow up on this action in the subsequent Retro. Consequently, a failure to actually make the proposed change should spark a Retro topic all by itself.
So, here are a few steps to turn your Retro into change happening:
Link cause and effectDuring the Retrospective, there should be a visible line between the input which sparked the proposed change and the change intention. As the Scrum Master, unless the line is blatantly obvious, do not hesitate to iterate the journey verbally: "We discovered that ... which leads us to ... and we hope to address this by ..." . Not only will this increase clarity in the team - if you get it wrong, now is a good time to clarify this.
Be clearState precisely what you want to change, why you want to change - and what the desired outcome of the change is. Likewise, define who should take the action item and how you will know if you got your intention. As Scrum Master, do not let the team "get away" without having an agreement on these teams.
Follow upThere is nothing worse than having volatile change initiatives without impact. Somewhere during the Sprint, you should have taken the discussed action. And hopefully, there are already results in the next Retrospective. As Scrum Master, encourage the team to share the effect of the change during the Retrospective before digging into new items.
Learn from actionProvide a forum to discuss the impact, celebrate success and acknowledge learnings from failure. This encourages team members to make the next adjustment, because they will feel that the Retrospective is valuable.
If the change failed, be open about it and discuss whether you want to reverse to the former state or try an alternative route.
If the change was impeded, you can already use that as an entry point for discussion: What can we do about that, how can we be more successful next time?