If you have a better analysis package available to you, why are you using STAAD? STAAD is OK for stiffness analysis. However, it often does a poor job at dynamics. Its solver is inferior.
Your "lower" modes should be your primary modes and should have high mass participation. If your lower modes have small mass participation, they are probably local modes and not global modes. The lower modes may be local vibration such as a beam in a floor (low mass participation) with...
