[lammps-users] CNT modeling

Dear friends,
I have a question. As I put CNT under tension with a constant rate e.g.(erate=0.2), the nanotube extends about 30% and after that it’s length does not change anymore. The same thing happens for (erate=0.1), but the nanotube extends only about 5% and keeps that lenght in spite of box extension.
When I checked the box size, I saw that the box size is changing as it should but atoms do not move after those strains. I am using “fix deform” command and it is said that the atoms inside the box move proportionally to the box size.
I want to ask to know why the atoms do not follow the box extension after a while?