when doing mixed mesh analysis SolidWorks Simulation doesn't always assume that the two different elements are bonded.
For example, if you have beam mesh and solid mesh together, you must specify a bonded contact condition between a face of the solid and the bea.
I think you should try to follow the SolidWorks Simulation tutorial for mixed meshing which is installed with the software. I don't know the name in English because my software is in italian, but it's the one that is shown in attached picture.
Tutorial.jpg 14.8 KB