Why can't I scale up jam/jelly recipes?
I haven't made jam in years and don't know first-hand, but this website suggests it won't scale as the jam won't cook as well.  This seems predicated on the idea that your cooking vessel remains the same size but the batch is larger.

From the linked site:  


  Most jam recipes already call for you to use the widest pot you have, for maximum surface area. This large surface area leads to faster evaporation of water. Fast cooking leads to the freshest tasting, best textured jam.
  
  However, if you double the amount of jam in your pot, you greatly increase the cooking time, because there’s so much more product in the pot that needs to be cooked down. This can lead to rubbery batches, burning and jam that doesn’t set. 


This is also supported by another site that claims doubling is difficult "due to inherently uneven heating of home cookware".

Per nico's comment, it sounds like doubling/scaling is possible - perhaps give it a try in cookware appropriate to the doubled batch size.