My gelatin didn't gel
As the instructions on the box say, you shouldn't put fresh pineapple (or kiwi-fruit) in the Jell-O. Apparently pineapple has an enzyme called bromelain that breaks up the gelatin into its component amino acids.

You can use canned pineapple instead as the pineapple is cooked during the canning process and this denatures the bromelain.