Missing part of structure

Hi, everyone!

I found that part of my structure (the tail) was seen in the 2D view after ab initio 3D reconstruction, but is missing in the 3D volume (please see the attached images). The tail region did have lower intensity in the 2D image. But the region was nowhere to see even if I lower the contour level in Chimera. Is that because this tail region is too flexible?

Thank you!