Problem during printing, under extrusion

Vertex 3D Printer Vertex - Issues while printing
1

IMG_20190718_170837-min
IMG_20190718_170837-min4160×3120 848 KB
IMG_20190719_102805-min
IMG_20190719_102805-min4160×3120 751 KB
IMG_20190719_102812-min
IMG_20190719_102812-min4160×3120 457 KB
IMG_20190719_102829-min
IMG_20190719_102829-min4160×3120 511 KB
IMG_20190719_102841-min
IMG_20190719_102841-min4160×3120 473 KB
IMG_20190719_102849-min
IMG_20190719_102849-min4160×3120 555 KB

Hi all,
I have a k8400 with an original e3d v6 extruder. During the stama, the filament stops coming out and then starts again. Attached some examples.

Know what it can depend on?

Thanks,
Michele