Jam Making

I make a lot of jam. I tend to know when it will set, when it won't, the different recipes you can fudge, the ones you can't. And rarely have problems with it not setting.

Of course, until I started making to sell.

I made 4 batches yesterday - 2 rhubarb and 2 rhubarb/blueberry mix (aka blubarb). I decided I'd make them all on the same day and really watch how I made them - being precise in my repetition to see if they'd all jell the same or not. The results I got were NOT expected.

There were jars in each *batch* that didn't set. What?! How can you have the exact same jars that were prepared at the exact same time in the same manner not set with the same batch of jam? It blows my mind. So, I have some jam, and some syrup. I'll probably try again with the syrup, just to see if I can get it to set, but WOW, who'd have guess THAT?

It's not me. It's not the pectin. It's not the weather. So what is it?