I thought I'd pass on what I've learned over the years about the main problem with jam - how to get it to set.
First off, like most things in life, especially in the kitchen, success lies in practice and patience.
The first is rather obvious - make as much jam as you can as often as you can!
The second one is a bit more complicated. Patience is required in many steps in the process. The first, and a very basic one that a lot of folk don't consider is to wait until the fruit is fully ripe - you can't make good jam with under-ripe and hard fruit. A few extra days of sunshine at this time of year can make all the difference. Patience when picking - treat all fruit gently to avoid bruising and crushing at this stage. Next is preparation - yes it takes time to de-stone plums, hull strawberries, top and tail gooseberries, strig currants - but surely that is all part of the pleasure, no?
Take time, if using a recipe, to read it thoroughly, all the way through to the end, more than once - make sure you have enough ingredients, the right equipment to hand, all clean and ready to go.
Let the fruit either stand overnight, or leave on a low heat for a while to soften, especially for fruit with tougher skins - blackcurrants, plums etc.
Use a wide pan for jam making - the wider area at the top ensures rapid evaporation of the liquid, resulting in a good thick jam, with a more certain set.
Give the contents of the pan plenty of time on a lower heat to thoroughly enough to completely dissolve the sugar; if it's not completely dissolved, this encourages crystalisation fo the jam - a thick sugary crust forming on the surface of the jam; whilst edible, not paricularly pleasant, and certainly wasteful.
Give the jam a long enough time at a high temperature to reach setting point. Most folk start out with a jam thermometer; the wrinkle test is very reliable - a small dollop on a cool/cold saucer, left for 30 seconds or so, will wrinkle on the surface when pushed with a finger to show the jam is ready. A third method, and the one I use now, is the sheeting method - lifting the wooden spoon from the jam, it will not leave the spoon in single drips, but they will join together on the edge of the spoon and form a sort of curtain effect before leaving the spoon edge back into the pan. It's wise to draw the pan from the heat whilst testing, as it will continue to cook and may over cook if time is being taken to test. A few minutes at such a high heat can make a difference.
I take my pan from the heat when setting point is reached and leave on the side for a good ten minutes or so - I put the jars into the oven while doing this. This gives the jam time to relax a bit and settle; if not done, the fruit will frequently rise to the surface instead of being evenly distributed throughout the jar.
Another tip is not to have the jam and/or the jars too hot - if the jars are too hot, the jam will boil as it touches the hot glass, and can impair the taste of the final product.
So - just a few pointers from my 20+ years of jamming and preserving, I hope you find them interesting and useful.
Happy jamming lol
ps - if your jam doesn't set, it is well nigh on impossible to re-boil to a satisfactory jam; I see lots of ideas for using unset jam as a sauce for ice-cream,but there's usually an awful lot of jam to get through! On the admittedly very rare occasions mine hasn't set, it's gone into the brewing bucket to transform it into a drinkable jam instead !