Problems with gelatine got the following recipe from a UK newspaper.  Tried it and it did not set.  At all!  My gelatine was fresh and 200-225 bloom and the grapefruit juice was freshly pressed.  It tasted delicious but we had to drink it!  Contacted the food and drink department of the newspaper but just got waffle as a reply.  Tried it again with double the gelatine but it still didn't set fully and had a slightly different taste.  Has anyone got any ideas please?

4 sheets gelatine, 
400ml pink grapefruit juice, 
150ml Campari, 
2 tablespoons sugar.
Normal method of soaking gelatine and adding to slightly warmed juice.
The problem is that you're using 3 things that are enemies of gelatin:


Acids
Enzymes
Alcohol


These 3 things are very common protein denaturant agents, which means that they break down the protein structure. And well, gelatin is a protein, so adding freshly pressed grapefruit juice (acid + enzymes) and alcohol to gelatin will prevent it from setting. 

You should boil well the mixture of sugar, juice and alcohol before using and rectify volume with water.