When showing data in grids over assocation or datasource microflow, the default search fields are not available. You would need to implement your own search functionality. 

Lennart Spaans has an excellent tutorial how to create such functionality. (Although this is specific for native, this will also work for responsive) https://medium.com/mendix/how-to-create-a-search-functionality-in-a-mendix-native-mobile-app-f6f48b59f1be