There are many different things that can cause wrinkles in a vinyl liner. Most wrinkles occur in the winter when the pool is closed and the water is lowered. When the hole for your pool was originally dug, it was done with an over dig a few feet larger than the pool itself. When the pool is back filled, the dirt used will not be as compacted as the dirt around it. This causes the hole to hold water for awhile before its absorbed into the soil around it. When the ground water collects and rises above the level of the pool, it can cause the liner to "float". {See this article on dealing with floating liners} Once the water is absorbed, the liner will go back down but not always snug to the surface as it was before therefore causing wrinkles.
Another cause of wrinkles in a vinyl liner after your pool as filled is poor water chemistry. Low pH (particularly pair with a high chlorine level) causes vinyl liners to absorb the water therefore stretching the liner, which leads to wrinkles.
While the only sure way to remove wrinkles is to drain the pool and reset the liner, this can be very expensive and dangerous. Draining the pool always runs the risk of structural damage. There are a couple of methods you can try before having to go to those extremes since draining your pool should always be a last resort.
Before you try any method, make sure you have a warm sunny day to ensure the vinyl is flexible. If you have a heater you can also heat the water to 92 degrees or above making it much easier to work with.
The first option, and maybe the easiest, is to use a toilet plunger. You would need to get into the water and plunge the area around the wrinkle. Plunging pulls up on the wrinkle causing the liner to spread and possibly removing it. Just make sure that you are using a clean plunger that wasn't previously used for other jobs.
Another option is to use a squeegee or brush. Start in the center of the pool and push the wrinkles toward the wall of the pool. Work them all the way to the top until you can adjust the liner in the track to its original position.
For an above ground pool you'll want to drain the water level down to just a few inches remaining. Get down on your hands and knees and push the wrinkles toward the wall all around the pool. Next take the top seat railing off the top and pull the liner taught untill its smooth in that section. Repeat this process all around the pool until the liner is smooth throughout the pool.
If you consistently find that you have high groundwater, you can prevent your liner from floating or wrinkling by installing a water drainage system around the pool. While this can be done before or after your installation, the easiest method is to use HydroBlox. You can learn more about HydroBlox here.
Always remember that the best option is to hire a professional to do it for you.
