Some commercial juices claim to be "100% Juice" however other ingredients are often added. If "other ingredients" are added, it can't possibly be 100%. Most juices like Ocean Spray, Northland, etc have Potassium Sorbate and often Potassium Metabisulphite. If preservatives are not used, shelf life would be extremely short.
Sulphites and sorbates will both hinder fermentation. I would recommend dumping the batch and start over with preservative free juice.
Several years back, I tried to make apple cider and it did not ferment even after I dumped 2 quarts of active slurry. The "100% apple cider", obtained from a local farmers market, contained potassium sorbate.