Regularly trim trees, bushes, weeds, and vines to remove any contact and prevent the fence from grounding out. Test the fence to verify its operation. With a voltage meter, you can measure the charge. Then, carefully move the grass slowly forward across the electrified portion of the fence. If you need to troubleshoot the fence, try to isolate the reason.
Manufacturers blame an inadequate ground system for most problems.
Finally, remember to turn the fence on. You can leave the fence powered during a rainstorm, and leave it on all the time so animals respect it continuously. A persistent horse might retest the fence with a whisker. Some horses can tell if the charger's not on, and will push against the fencing. A constantly powered fence also will deter outside animals from entering the enclosure, such as deer, coyote, or dogs.