Cascading Parameters Not Updating after Initial setting

Running SSRS in VS2012 on Windows 7.

My report has 3 parameters: Transaction Period, Transaction Start Date, and Transaction End Date.

The user is allowed to see and select the Transaction Period from a dropdown that's populated from the database (in the format of "2016-03", for March 2016).

The Transaction End Date is automatically set via a Default Values expression that expands the "2016-03" value out to a full date: 03/31/2016.

Finally, the Transaction Start Date is automatically set to a date 13 months prior to End Date via a Default Values expression.

The very first time I select "Transaction Period", the Transaction Start and Transaction End Dates automatically update correctly.  The next time I choose a different Transaction Period, the Transaction Start/End Dates do *not* update; they just remain the same.

Question: How can I get these parameters to update with each selection change in the Transaction Period dropdown?

Thanks.