Barbel spawning are activated by the culmination of a couple of factors,water temp,first decent rainfall etc.The timing of it is not a 100% exact science.It may be possible that barbel have finished spawning in one body of water and are yet to begin in another merely a mile away,mainly due to the factors above.
Spawning is also not the worst time to fish for them as artlure tends to evoke some aggresive strikes during this time,the comment being that "hy's seker kwaad omdat hy van die nes afgeval het" lol, also they tend to be closer to the side as spawning activity generally takes place amongst reeds and grass-patches.
It is always worth it to give it a go,during spawning,be it from the bank or boat.
Enjoy