Why hasn't my marmalade set?

[zisforzebra]

I've made marmalade today from a grapefruit, a lemon and 4 oranges using the whole fruit boil method. It rolling boiled for a good 30 minutes, hit 105 on the jam thermometer but it hasn't set at all. My lovely three fruit marmalade is bits of peel floating in sugar water at the moment.

Why hasn't it set and can I turn it around or am I left with two litres of ham glaze?

Mine has not set again ☹️ I am going to save for cooking (muffins/cakes/ duck a la orange)

So disappointed

I gave up its edible but very dark and think mostly only peel left. Normally have no probs. Could it be this years oranges.