When more than one waveguides are included in the port, due to mode degeneracy, you may not be able to find the case that each waveguide supports a specified mode. You will need to add a port for each waveguide, and make the port span as large as possible without creating mode from the adjacent waveguide. The port cannot just have the same size as the waveguide core, since the mode has tails. You may also need to confrim the mode is what you desire, by visualizing the mode profile. Or you can use "user select" and calculate the mode, and just choose the one that you expect.
If this is a slot waveguide, you just only need to increase the port span to find the slot mode.
In any case, you will need to justify the mode is correct.
I guess this is EME simulation.