Wait first: a set is not immediate
Before diagnosing anything, give it time. A pectin network keeps forming as the jar cools and keeps firming for a day or two afterwards, so a jam judged at bottling is being judged too early. Something poured warm and still slack the next morning is worth a second look on the third day before any decision is taken.
What that means practically: do not reboil on the evening you made it. Nearly every “my jam did not set” that resolves by itself resolves in that window, and a batch reboiled at the twelve-hour mark is a batch that has been cooked twice for no reason.
How to tell, and why the thermometer is the second-best test
The temperature that indicates a set is 220 °F (104.4 °C), or more usefully 8 °F above the boiling point of water where you are standing. The second form is the one to use: boiling point falls with altitude, and a fixed 220 °F is wrong by a measurable margin in a kitchen a few hundred metres up.
Even corrected, the reading is a proxy. It measures how concentrated the syrup is, not whether pectin has bonded, so it can be reached exactly by a fruit that will never gel, which is the whole failure mode of a low-pectin jam. The cold plate tests the actual thing: a spoonful on a chilled saucer, back to the cold for a minute, then push it with a fingertip. If it wrinkles and holds a furrow, the network is there. If it runs back, no temperature reading makes that untrue.
Which of the three faults it is
Short of pectin. The commonest, and the one boiling cannot fix. In this register 7 fruits are classified low: apricot, blueberry, sweet cherry, peach, pineapple, rhubarb, strawberry. A further group carry no published classification at all, recorded as not established. A jam of one of these thickens as it reduces and still pours when cold. It needs pectin from a product or a partner fruit. Rhubarb is the clearest case.
Short of acid. 14 fruits here measure low in acid: apricot, banana, blackberry, sour cherry, sweet cherry, clementine, fig, guava, mango, honeydew melon, orange, peach, pear, persimmon (japanese). These may have pectin and still not set, because the chains cannot bond at that pH. The correction is lemon juice, and it is the opposite of the first case: adding acid to a rhubarb does nothing, adding acid to a fig is the whole answer.
Not concentrated enough. The only one of the three that more cooking solves. The pan has not lost enough water for the sugar to reach the concentration at which pectin can hold a network. See jam sugar for the line this refers to. A cold plate that gives a soft, slow wrinkle rather than none at all is usually this one.
Whether to reboil, and what it costs
Reboiling works when the fault is concentration and when it is a missing ingredient you can now add. It does not work as a general remedy, and it is not free: every additional boil darkens the colour, dulls the fresh fruit flavour and drives the batch further towards caramel. A preserve reboiled twice tastes like a preserve reboiled twice.
Which leaves a fourth answer that is not a repair at all. A loose preserve, somewhere between a compote and a jam, is a legitimate product rather than a failed one, and a great deal of home-made jam is exactly that. What matters is choosing it rather than arriving at it after an hour at the stove. The full per-fruit figures are in the register, and the mechanism is in the preserves section.
Questions about a jam that will not set
How long does jam take to set?
It keeps setting as it cools and keeps firming for a day or two after bottling, so a jar judged at bottling is judged too early. Wait until the third day before deciding it has failed.
What temperature does jam set at?
220 °F (104.4 °C), or better, 8 °F above the boiling point where you are, because boiling point falls with altitude. It is a proxy for concentration, not proof of a gel, so the cold plate is the real test.
Why will my jam not set?
One of three things, and they need different corrections: the fruit is short of pectin (add pectin or a partner fruit), short of acid (add lemon), or the pan is not concentrated enough (boil longer). Only the third is fixed by more cooking.
Can I reboil jam that has not set?
Yes, and it helps when the fault is concentration or a missing ingredient you can now add. It will not conjure pectin the fruit has not got, and every reboil darkens the colour and flattens the flavour.
Will more sugar make jam set?
Not by itself. Sugar is one of the three conditions and does its work by competing for water, so a pan already at concentration gains nothing from more of it. Without pectin there is nothing for the sugar to help.