CPWs are tricky.
your frequency range is not that small... it is huge.
separate dielectric and air region around feed points and CPW slots, seed them with denser mesh. You can just split larger objects with a box defining the region of interest there. If not doing this, HFSS may make a mistake by skipping to a wrong mode, or even skipping gaps completely.
use only discrete sweep, preferable with multiple meshing frequencies around the band.