I used the sugar free jelly crystals like I have before, and they all dissolved properly. Might it be the fruit I put in? (Kiwi, raspberries and mango).
It'll be the kiwi. It has been known to stop jelly setting other than that maybe the jelly was out of date. I had this problem once as didn't realise I'd had the jelly for so long.
it does state on the packages what fruit exactly wont make it set, just so you know every other fruit not listed on the back is fine, and yummy too.. sorry about your jelly hon! x