Cause: You defined mass source from load patterns but forgot to assign any load pattern to the roof diaphragm. Or, you used a rigid diaphragm but the beams are not connected to the diaphragm constraint.
Before you finalize your ETABS model for seismic design: etabs mass summary by story
After you verify the mass summary, run a modal analysis. The Mass Participation Factor report depends entirely on the Mass Summary. Cause: You defined mass source from load patterns