As you can tell in this picture the fence is ready to fall over. When you see a few posts all in a row like this that are falling over and the rest of the fence looks good this means that there is rock here and whom ever installed it did not drill out the rock and concrete the holes.
The installation of the post is even more important when you are using polymer. The terminations (or any where the polymer ends at) needs to be concreted and braced. The terminations in polymer are the most important aspect of the fence installation, simply because the entire pressure of the fence is really held by the posts in the termination. If the end posts are not installed, concreted and braced right they will pull over from the tension of the polymer. The corners also need to be concreted but to a lesser extent since they do not have nearly the pressure the terminations have.