It might be a terrain issue. Attacking into a square uses up however many movement points that the tank would spend moving into that square. If the defenders are on a hill, tundra, forest, or jungle (or fallout), all of the tank's movement points will be expended after just one attack.
Otherwise, blitz for tanks has been working correctly for me (currently using 3.13 patch). I have not had the chance to try blitz on naval units.