A Motion Analysis study eliminates redundant mates when calculating motion. This results in a calculation of zero force at the mate locations of the affected parts. You can have SOLIDWORKS Motion automatically replace these mates with bushings. Forces are then calculated at the locations of the redundant mates.
To address this, select Replace redundant mates with bushings under Motion Analysis in the Motion Studies Properties PropertyManager before running the study. This leads to better force results in most cases.
After computing the force results, you can edit the overall bushing stiffness and damping values used in the motion analysis.