First a few comments, but be sure to read all the way to the end. Sealing the jars in a water bath has nothing to do with whether the jam sets or not. Setting is a function of cooking the fruit to the right point, either with or without additional pectin, so that the sugar and pectin react and begin to set. Pectin is a natural carbohydrate present in fruit that enables it to gel. Cherries are notoriously low in natural pectin, so most recipes advise you to add either a store-bought pectin or a home-made pectin mix made from apples. Ripe cherries (like most fruit) are lower in pectin than immature cherries, which is why many canners use a mix of 75% ripe fruit with 25% not yet ripe.

There are a number of circumstances that affect whether or how well your jam sets. If your jam does not set, chances are it contains too little pectin. It’s also possible there is an imbalance between the pectin and the acid in your jam. Adding a little lemon juice helps the pectin, and also helps create an environment hostile to bacteria. High humidity in the kitchen can cause problems with jam. Finally, you can defeat the whole purpose of adding pectin if you boil the mixture too long - overcooking causes the pectin to break down and lose its thickening capacity.

People have different perspectives on fixing a jam that does not set. Joan Hassol, author of Well Preserved and owner of a jam business of the same name, doesn’t like to add more pectin to a soupy jam. She dumps it all back in the pot, adds a little sugar and lemon juice, and reboils. If it doesn’t set up the second time, she says, she labels it "Pancake Sauce," and moves on to other projects. Other sources advise you to add more pectin and bring the jam to a boil until the setting point is reached.

But a greater concern is what has been happening to your cherry jam in the past two weeks. Since you did not process the jars in a hot-water bath, you sidestepped the primary mechanism for preserving the jam and keeping it safe. A boiling-water bath heats the jam, jars, lid, etc. to a temperature that kills the bacteria, yeast, and molds that cause the food to spoil. More importantly, a boiling-water bath is adequate with high-acid foods (like fruit jams) to kill the bacteria that causes botulism, which otherwise can thrive in the vacuum of a sealed jelly jar. (Non-acidic foods need to be heated under pressure to reach a temperature of 240° F (115°C) to be safe.)

If your jam hasn’t been kept in the refrigerator since it was made, we would be cautious about keeping it at all. It would be wiser to try to fix a sloshing jam right away, and then to go through the process - however laborious - of preserving it in a water bath.