We are in the process of putting up an 18ft above ground pool that we bought used. It is 2 years old and we took it down ourselves. We were very careful with everything and bought a new liner. We leveled the ground and have the pool tracking and wall in place... on one side I have noticed that the wall keeps popping out of the track and we push it back in and with in a few minutes it will be worked out again. Does anyone know what the problem could be? Do we need to take it all down and start over? Or could it be that it is not in a perfect circle somehow?

No, you do not have to start over. However you have a serious problem which MUST be corrected. Perfect circle is not the issue, perfectly level is. Your track is out of level but if you take only a few measurements in certain points it may appear level to you. Use a water level or transit and move contiunously around the track, it pops out of the low spot. Tell me more about the base under the track before we even think of a cure.

The base under the track is firm hard dirt. We used a 4 ft level taped to a 10 ft board to go around in a circle to check for levelness. My brother brought his tractor over and took the highest ground out and then we leveled by hand the remaining little bit. So do you think the opposite end of the pool is where it is the lowest and this is causing the opposing side to pull upwards? I am fearing that we will have to take down the walls and start all over. :(

The entire floor/bottom of the pool doesn't have to be perfectly level and flat but the ring under the track must be perfect. From what you have said so far it seems that, at most, you are only 1-2" from flat. Notice I said flat instead of level, you can be flat and still be out of level, we'd like to get both here. Put your 4' level back in the tool box, the way you tried to use it can easily allow you to be 3-4" out of level, this is what you need to build YouTube - ‪How to Build a Water Level‬‏