dude i know what you're talking about ... i always think my way is the right way to beat the game ... i have nothin' else to do but ive beatin' the game seven times and it happens to me where they wont kill the tank ... so try this
try going back to the beginning of the tunnel and there should be a ammo box it should restore some health and then advance and tell your guys to attack ... sometimes you'll go past the tank and they wont advance ... you gotta move around ... yeah i know it sucks but that should work ...