First of all, make sure there is nothing in the liner track that will prevent you from getting the liner in properly. Then you want to wait for a nice warm day and then get some almost boiling water. Slowly pour the hot water on the liner to make it more pliable. Only do a foot or so at a time. Be careful not to melt the liner. Once the liner is more stretchy, pull it up and into the track. Once you've got a few inches in there, pour cold water onto the section to cool it down and reduce its stretchiness. If the problem continues, get yourself some T-Bead or Liner-Lock from a pool store and insert it as soon as you get the liner into the track. You can also use a hair dryer for heating the liner.

Brad
Waterworks Pools