It does not matter at all what type of symbols a machine has, such as wild chrries, bars, double bars, double diamonds, 7s, and so on. What does matter is how many reels a machine has and how many symbols are on each reel. The more reels and symbols, the harder it is to hit the jackpot or even a winning combination.

For example, if you need to line up three gold 7s on one payline to win the jackpot, it’s likely that each reel only has one gold 7 on it. On a 3-reel machine with a total of 18 symbols on each reel, your chances of lining up all three gold 7s are

Possible symbol combinations on the three reels: 18 x 18 x 18 = 5,832
Odds on hitting three gold 7s on three reels are: 1/5,832 (1 in 5,832)

It gets worse when you play a 5-reel slot machine with 18 symbols each:
1/ (18 x 18 x 18 x 18 x 18) = 1,889,568 ( 1 in 1,889,568 )

the number of reels and the number of symbols on each reel greatly affect your chances of winning. The more symbols, the little chance you can hit a winning combination.