It's working now. I may have fell victim to request caching in my browser. I was previously using MVC for the DataSource, which apparently uses a POST for the request parameters. I changed to Web API, but last week the parameters were still being sent in a POST. When testing today, the request is sent as a GET and all parameters are getting through. I assume the different request types for DataSource (POST for MVC and GET for Web API) is by design?