A common error is to use an STL file with stretched triangles which are removed below a small angle tolerance. Check the standard output to see if you get any message such as:
STL file(s) successfully read.
Total number of facets read = 208
Number of valid facets = 144
Number of ignored facets = 64
If there are ignored facets, either fix the STL file outside the GUI, or lower the small angle tolerance until there are no ignored facets.
Another common issue is to have the STL normals pointing in the wrong direction. Please see https://mfix.netl.doe.gov/doc/mfix/19.1.4/reference/faq.html#how-to-verify-flip-stl-file-facet-normals for more details.
If particles are too soft, reducing the fluid time step won’t help. You need to increase the spring stiffness for particle-particle and particle-wall collision.
If you get stuck too long, feel free to post your .mfx and stl files, and someone may be able to provide a better solution.