Just wanted to add - I looked at the leaflet in the box which gives sample recipes. The amount of juice is different depending on the fruit, and varies from 3 1/2 C (strawberry) to 7 cups (apple or crabapple). Amount of sugar varies too. So I think your recipe is okay, it is just for a fruit with little or no pectin.
For strawberry they say 3 1/2 cups juice, 5 cups sugar to a box of pectin.
I've experimented with herb jelly too (zero pectin in that!) and the recipe called for 3 cups prepared juice to 4 cups of sugar. (The mint jelly from this recipe was kind of horribly sweet to my taste).
There's another leaflet in my recipe book, from a different pectin brand, gives a recipe for Elderberry: 3 cups prepared juice, 1/4 cup lemon juice, 4 1/2 cups sugar, 1 box pectin.
Anyway the higher amount of sugar in your recipe will help it to gel, if anything.
BTW, front of the Certo leaflet "Tips for Jam Making Success" it says "Always check the 'best before' date before you begin. Sadly, I had to learn this the hard way.
(or should I say, the soft and runny way
lol
Luckily as Rajun said, no worries, just a load of extra work to boil it up again with more (or non expired!) pectin.