From any open schedule or skeleton in MSP, click on the Scheduler menu at the top of the window and select "Scheduler Settings...". In this dialog, make sure to set the option to, "Never exceed a volunteer's 'x times per week / month / year' preferred serving frequency".
The "once per month" serving frequency does not guarantee that the volunteer will only be scheduled once in a given calendar month. It means that in a two-month schedule, for example, they will not be scheduled more than twice. The scheduler will space out their services reasonably.