1. You probably used the flamethrower too much in the earlier levels, so its effects on the Alien were diminished by the time you reached this level. 2. The attack was already initiated and the Alien couldn't be pushed away at that point, regardless of the method. I played the game twice and specifically tried to use the flamethrower as little as possible before this level, so I could have an easier time in the nest.
1. You probably used the flamethrower too much in the earlier levels, so its effects on the Alien were diminished by the time you reached this level.
2. The attack was already initiated and the Alien couldn't be pushed away at that point, regardless of the method.
I played the game twice and specifically tried to use the flamethrower as little as possible before this level, so I could have an easier time in the nest.