intermitent failure of freezer jam to jell...same fruit, another surejell box

I have used sure jell for many years to make my freezer jams. I have always been puzzled as to why, when making up any type of jam this problem crops up:

Using the same batch of fruit, usually requiring 6 or more boxes of sure jell....out of the half dozen boxes there will be at least 2 that instantly look like there will be no jelling of the jam. I can tell immediately either upon stirring the pectin into the water ir observing the mixture as it comes to a boil because it stays thin and watery even if I boil it longer than one minute!

Does anyone know why this happens AND....
can I open another (expensive!) box of sure jell and mix it either into the same hot mixture or into the fruit that has already received the failed surejell mixture?

Any advice will be appreciated....I'm positive I followed directions to the letter and measurements are accurate because I'm constantly aware of what happened the year before and I end up with runny syrup instead of jam...!