If jelly is too soft, there may be incorrect proportions of sugar, acid and juice. Making too large a batch or undercooking may also be a cause. Another problem is jelly that weeps. If this happens there may be too much acid in the fruit or too much acid added in the form of lemon juice. Also, the storage place may have been too warm or the temperature changed too much during storage.
Fermentation is another problem.
This may occur because of too little sugar or improper sealing. The boiling water bath process helps to get an adequate seal and prevent fermentation. Sometimes jellies and jams darken at the top of the container. Red fruits such as strawberries and raspberries are especially prone to fading. Or, you may have allowed the jelly mixture to stand before it was poured.