No density in the center of map after homogeneous refinement

Dear cryoSPARC users,

I got a problem when using homogeneous refinement or heterogeneous refinement. I found the density is very low in the center of the map (I attached the half map). It’s almost empty inside of the map. My protein is ~ 120 kDa. I used most default parameters except changing “Maximum resolution” to 6 A and “Initial resolution” to 15 A during Ab-Initio reconstruction. It happened in almost all of my refinement. Any suggestion or advice will be really appreciated.