[Webmonster]

My  peach jam did not set well. I used 3 bags of sure-jell for 12 cup recipe.  Can I reprocess the peach mixture adding a thickening agent. With clean jars and new seals?

[Yorkie]

Welcome to the site 

I'll move your question over to the Cooking board; feel free to pop back into the Welcome board to say hi properly. 

[DanielCoffey]

I had a similar issue when I tried nectarine jam a few years ago. What I used it for in the end was as a loose topping for natural yogurt.

If you re-boil it, the jam will probably become dull in appearance and get a slight "butterscotch/caramel" taste which will detract from the peach.

List the recipe so folks can have a look.

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In the meantime, here is the recipe that is in the Marguerite Patten book (she prefers nectarines over peaches for this)...

1 1/4 lbs peaches (skins slipped off in hot water then cooled)
2 tbsp water
2 tbsp lemon juice
1lb white sugar

Makes 1 2/3 jars