Hey! Stop hating on me.

All Gym Leaders are tough,
duh! Don't make me cry...
Seriously, it's probably because as Draco said, Gym Leader Pokemon have higher stats than normal... I mean, they're supposed to be a boss. Though, if you feel like your Pokemon still should be doing more, make sure the moves you're using aren't the newly-implemented ones that don't work. Most of them don't do anything except cause a small amount of damage. It's because they're placeholders.