how do i tell blender to let the knife cut though non vivible as well

I got out of wire frame so the visible icon would appear (that is a daft restriction!)
but the knife still only cuts the front of a tube.

Look at the bottom of the viewport after you pressed [K]… Blender talks to you. :wink:

In plain English: Press [Z] to cut the whole mesh. (And you don’t need to see in any way what you cut. Wireframe mode or selection not limited to visible aren’t necessary.)

AHA! thanks for your help