I make a few wooden stakes (like tent pegs) to pin the fence to the ground in places where in doesn't touch quite tight enough. The bottom line isn't charged on most brands of fence so any gap between it and the ground is an easy out for the birds but also an easy IN for predators. Anyway... pin it down tight and then check your charge. I've also found that the shorter lengths of fence (fifty feet) with the stronger two-prong posts tend to hold their shape better. I have one old 160 foot piece of fencing I call "The last resort". I hate moving that thing.
Also, I discourage the birds from escaping by making an example of them when they do get out. If I catch them, then I'll hold them against the ground (outside the fence) and mess up their feathers a bit. I'm not talking about beating up a chicken here. Just run your hands over them vigorously until they're a little pissed and a little shaggy looking. If possible, make sure your rooster sees it. It'll encourage him to disallow his flock leaving the fence area. Once done, toss her back over the fence and walk away. This seems to let them know that inside the fence is the safe place. Outside... not so much. The rooster will likely scold her and then he'll start being a bit more diligent with escapees. The hen will spend the next few minutes straightening her feathers and trying to figure out how to avoid that again. Most eventually get it. Some don't and become hawk and fox food.
Once they figure out that they can't go under, they'll try to go over... be ready for that. Might need to clip the feathers on one wing to put them off balance when they try.
I suppose my situation is likely different than yours but I figure if the chickens want out I'll let em wander, the fence is more predator deterrent and has worked very well on my dogs of which only one was ever an issue previously but now both steer clear of the fence so I hope that it deters predators just as well
However my chickens are still so small they can easily squeeze through the fence squares as if it weren't there so I have mostly not paid any attention to wandering chickens