I haven't had to use a fence charger in many years, so things may have changed, BUT:
I once had a QH mare that would break thru anything ---- until I bought the strongest, long range, cow fence charger on the market for her five acre pasture and made absolutely sure to keep the dirt around the ground bar soaking wet at all times.
Once or twice of being set back on her pretty Buckskin hind end finally cured her.
You DO have good ground (and wet dirt) where the ground wire is hooked to the bar ? Yes????
If not, there's the possible answer. Keep the ground well soaked to be sure the current is going thru at it's maximum voltage
You could also put a volt meter to the fence to see if the charger really is putting out the max you were told it would. But I would still go to the Co-Op and buy a charger strong enough to keep the Bull behind bars when the cow next door comes in season - lollollol