This really depends on what you are uncovering during your retrospective. If your team has little to say, it may be because they are a mature team who have been delivering together and ironed out many process issues. However, it may also point to an issue within the team itself or the facilitation method. If its the former, suggest you seek feedback from the team about dropping the frequency to every other sprint. If its the latter see if you can have an independent sit on your retrospective to give you some ideas on what could be wrong.