If you can post your file, that would help.
A couple of pointers:
- Sonnet uses a metal box, so the four sidewalls are perfect metal. Thus, you need to keep the sidewalls a very large distance from your antenna.
- Keep the cell size less than 1/20 wavelength. If this is a single patch, then set the cell size to be an exact fraction of the patch dimensions (e.g., if you patch is 123.4 mils by 567.8 mils, set the cell size to be 12.34 by 56.78 mils or half that).
- Set air layer above patch to be 1/2 wavelengh in air.