Too stiff or lumpy jam
If gel formation is too strong, due to way too much pectin, the jam becomes stiff, lumpy or granular in texture.
Cooking too long, but not at a high temperature, can boil off water, without breaking the pectin down. This results in jam that is too stiff.
This also occurs if the temperature is too high, for too long, or the jam is not stirred frequently.
Using underripe fruit, which has more pectin than ripe fruit, with the same amount of pecton as the recipe requires for ripe fruit, also makes stiff jellies and jams. FYI, commercial pectin is intended for use with fully ripe (but not overripe) fruit.
Runny Jam
Undercooking (it must hit a full rolling boil for ONE minute) or too little pectin or sugar leads to runny jam.
Overheating - that is too high temperatures or uneven heat distribution builds excess heat which causes the pectin to break down. This is why you shouldn't double batches - due to inherently uneven heating of home cookware - commercial canning equipment is design to heat more uniformly.