If you have played call of duty world at war and completed it then plz help me?

im on the very last part of the last level of the game in the heart of reich where u have 2 destroy the 3 flaks well ive destroyed 2 of them and ive seen form several you tube videos exactly how its done , but my game never seems 2 do wat it looks like in the videos, lik wen ur at the steps wen german slodiers keep coming, they are meant 2 go bak and then my men r meant 2 go up the staris with me, but they just stand on the ground like i will do all the work, im at the top of the steps and the germans just keep coming there r a lot of them but they arent still meant to running forward and in the actaul game ply my men r at the top of the steps but it deosnt happen wen i play it and then that buolding is meant 2 collapse but it doesn ;t and i was crouching and shooting continuously for like 10 15 mins but then i keep dying p.s. i do have the sniper and i know about the panzershrek but it still doens't make any sence and i did that level like 30 times and still couldn't do it it would be really helpfull if any one can help me or remeber wat to do