A.T. Robertson Commentary Mark 6:52

"for they understood not concerning the loaves, but their heart was hardened." — Mark 6:52 (ASV)

For they understood not (ου γαρ συνηκαν). Explanation of their excessive amazement, viz., their failure to grasp the full significance of the miracle of the loaves and fishes, a nature miracle. Here was another, Jesus walking on the water. Their reasoning process (καρδια in the general sense for all the inner man)

was hardened (ην πεπωρωμενη). See on 3:5 about πωρωσις. Today some men have such intellectual hardness or denseness that they cannot believe that God can or would work miracles, least of all nature miracles.