Hi,
I am not sure where and how you have tried to change filter data, but it is my understanding that .FilterText property is just a textual representation of the applied filter.
Highlight the required grid object in your tested application using Object Spy and check the Methods pane in the Object Browser / Object Spy. This pane will list all methods that are accessible from test script code.
Talk to your developers and ask them how they are setting filter in code. Check if the same method is accessible to you in TestComplete. Depending on the implementation of the grid, you might be needing debug version of it (along with debug information) to be used in the tested application in order to access required method.
