If the jelly doesn't  harden, there are several reasons.

(1)The gelatin didn't dissolve properly.

(2)Did not cool it in the refrierator enough.

(3)You are using fruits that contain proteolytic enzymes with the gelatin.

Some fruits (pineapple, papaya, kiwifruit, passion fruit, figs, etc.) and ginger contain proteolytic enzymes. If you use them raw, they will break down the gelatin and won't harden.  Same thing will happen with  raw fruit juice . If you want to use these fruits, use canned ones or boil them before use.

(4)Fruits with strong acidity are used.

Fruits such as lemon, orange, and plum juice break down the gelatin and may weaken the jelly.  If you are going to use sour fruit juice, adjust the amount of juice.

(5)The amount of gelatin is extremely low.