This will depend on the game. Some slots only give you access to progressive jackpots if you play the maximum bet or over a certain level. Others let you win at any bet level. Proportionally, you will have a better shot at winning the more you bet.
Reel slots will frequently pay jackpots only for the third/fifth line.
Whenever you play a slot with a jackpot, either online or live, it pays to double-check the rules.