Slitherine

I'm trying to protect my arty so I place two/three units in front of it. The space between my protecting units (infantry or tanks) is one hex. The enemy just goes around my "wall". am I missing something about ZOC I thought it was all the hexes surrounding a unit and once the enemy enters that hex it stops movement. Am I wrong? If i am what's the most effective way of protecting arty?

Recons (and any other unit with Phased Movement) can move past ZoC, so it's not safe to leave gaps to vulnerable units unless you're sure there are none to enemy units that can reach them.