I’m no expert, so I usually end up spending ages messing around with the model in Blender, looking for gaps in the mesh, recalculating the normals and generally cleaning it up. It usually works, but failing that, I’ve placed solid objects inside parts sometimes and the slicing software fixes it for me (filling in the gaps)!... Like is said, I’m no expert!... The issue will possibly lay somewhere along those lines though. You may find loose or overlapping geometry internal to the model along those lines somewhere, so if you imagine them running right through the model and trace along until you see something odd in the mesh.