Either you didn't kill the right mobs, you didn't kill enough mobs to ensure you would get at least one. Or plain and simple. You just got unlucky? It's a game of percentages. The only reason it's easy to get loads is classes with AoE skills can round up and take down entire camps of enemies which increases the chance of more DB mobs spawning. I'm a Dende and got 15 in the second event. That's with only 1 AoE at my disposal.
You have to pick a nice area and farm over and over again so you find as many DB mobs as possible and hope you get lucky. You could kill 100 and not get a single ball or you could kill 6 and get 7, It's percentage based. not a set "you kill this many you get one"
if i was going to tell you how unlucky i am in this game there is basicly the smallest thing i could find challenging. if everything is based on luck in this game i might as well just give up because worse luck then me is impossible to get