If too much pectin is used, the gel formation will be too strong causing the jam to become stiff, lumpy or granular in texture.
Jam can be too stiff if it is cooked too long at a lower temperature since the water boils off and the pectin isn't broken down effectively.
Jam that is cooked too high for too long or not stirred frequently enough.
Using under-ripe fruit in the recipe, which has more pectin.