OptiGrating will calculate interaction between selected modes. The input mode is selected by default, so no user input is required to see reflection/transmission spectra on that mode. On the other hand, tilted gratings are usually transferring power to cladding modes and evidence of that transfer is seen as loss in the transmission of the input mode. There are often many cladding modes, but OptiGrating will not consider any of them unless selected. The selection is made in the dialog box that presents the modes. Easy to miss, I know… 🙁