Unfortunately following a recipe to the T has different meanings for different people and can only improve with experience. If you have an instant read thermometer (especially the probe type), use it next time to see if you are really getting a hard rolling boil -- temperature should reach 220°F or very close to it. Failure to get and sustain that high temperature prevents the chemical reaction of gelling to occur fully, though it may eventually occur days or weeks later. Sometimes patience is the best recommendation.
Recipes using pectin require less hard boiling, usually only for a 1 minute period, otherwise you use the cold plate/spoon process (pg 29 of the Ball Blue Book) to find the gel stage.
I would anticipate that your jams might have been runny to begin with and needed to cook longer to evaporate some of the moisture first before going to the final step of setting it up for gelling. That, and temperature, are two of the main biggies you have to pay attention to.
Even experts gets 'syrups' at times. They are good on waffles, pancakes, oatmeal, etc. or you can keep some 'Instant' ClearJel® on hand to add when you open a jar to thicken it up.
Support bacteria. It's the only culture some people have.