Passage: Gen. Arnold finally began his advance toward Ormoc with a novel tactic. On the night of 4 December, vehicles of the 776th Amphibian Tank Battalion put to sea and leapfrogged south along the Leyte coast and positioned themselves west of Balogo.  On 5 Dec., the tanks moved to within 200 yd (180 m) of the shore and fired into the hills in front of the advancing 17th and 184th Infantry. This tactic proved effective, greatly disorganizing the defenders, except where ground troops encountered enemy pockets on reverse slopes inland, shielded from the offshore tank fire. The 7th Division pushed north with two regiments which encountered heavy enemy fire coming from Hill 918, from which the entire coast to Ormoc City could be observed. By 8 Dec., the American forces had taken Hills 918, 380 and 606, plus the surrounding ridges. By 12 December, Gen. Arnold's lead battalion was less than 10 mi (16 km) south of Ormoc City.

Question: when did the tactic not work?
where ground troops encountered enemy pockets on reverse slopes inland, shielded from the offshore tank fire