loom contains significantly less cells than those in "filtered_feature_bc_matrix"

Hi, I found the loom files generated from loompy contains much less cells (80% less) than those in "filtered_feature_bc_matrix". This is quite significant. I also noticed a warning in the standard out file during the "Calling cells" process: "WARNING - No ambient RNA beads were found; maybe sample had too few cells?". How do I save this problem?