Parameterized data source bound to field

I want end users to be able to choose between pulling in test data or prod data in the report. I have a test and prod SQL server. Inside are identically named databases and schemas, so all I need to switch is the server name. I've successfully created a parameter that defines the server name, and all works fine when I change the parameter via the usual method ("Edit Parameters") in desktop (haven't yet tested in Service).

However, I've also created a table with the possible values for my sql server parameter, and bound that field to my parameter. When I throw my bound field into a slicer, nothing changes in my data, and the parameter value doesn't change either. Am I missing something here?

If it's relevant my datasource is connecting via Direct Query.