Be careful when you tighten the ring on the jar and lid. . .
You want to set the lid right from the hot water it has been sitting in onto the hot, filled jar then apply the ring and tighten ONLY finger tight. That is ONLY as tight as you can get it with the tips of your fingers. It is possible to over tighten the rings and thus make the lids warp or buckle enough to lift the sealing compound off of the jar and this is how you may loses a seal when the jar cools or the jar may never seal at all.
Plus, the proper head space on jams and jellies should be 1/4 inch.
Ultimate seal test: Wait 24 hours without touching the jars then make sure all dome lids are indented down towards the inside of the jar THEN remove the ring and pick up the jar by ONLY using the lid. If you did everything right, that seal will hold, even on quart jars.
Also, wash your jars to make them clean and pretty, label them with the date and product. Leave the rings OFF for storage. A clean, ringless jar will show any fermentation of product that may bubble over and leak from a jar that has lost the seal. Plus the jars look gorgeous.
Do all of the above and then water-bath for 10 minutes and you will be safe to store in a dark cool place for at least a year. You will NOT need to refrigerate if you are using high acid fruit (strawberries and lemon juice are high acid), sugar and a boiling water bath. The acid in the fruit helps prevent to growth of many organisms, the sugar prevents organisms from getting the water they need for growth and the heat destroys many organisms.
And if it does not jell, you can re-process or just call it syrup or ice cream topping!
mistral, Master Food Preserver, at your service.