Bungees stretch over time and eventually need replacing, though sometimes you can simply tighten them and get a few more years.
Lou was paddling my daughter's boat at Arroyo Colorado, a 10' Redfish. I had added a skeg years ago, because this short boat needs a keel extension for tracking downwind in a strong coastal wind. It really makes this boat into a better than decent coast ride, because it's already a wind-cheating hull design.
The skeg has a rigid cord to retract it, and a 3/16" bungee wrap around a drum to deploy it. When we were coming in, the skeg wouldn't deploy for Lou - not too much annoyance, he turned around and pushed it with his paddle, and it worked fine from there - he got home.
I found the culprit when I got home - the capstan bungee was frayed and showing bare rubber. I had a piece of the right stuff in the dry bag, and replaced it.
Whenever you replace bungee cord and cut it to length, light the end until it burns like a candle, let it burn for a few seconds, then blow it out. Don't touch it - let it solidify and cool. Do the same to the leftover piece that you're going to store until you use it later.
My boat was suffering two 1/4-inch bungee ailments
First, the bow bungee was floppy, and wouldn't secure my bow line, which was the only thing I was counting on it for.
Second, the sternwell bungee was doing a great job securing my milk crate, but was strangely floppy where I wanted it to secure my dry bag.
The bow was easy. I simply pulled four inches at the end knot, tied a new knot, clipped and burned the end. Close inspection showed there were no other concerns, and this bungee will last me a few more years.
Messing with the long sternwell bungee, it was still working great securing the box in the front half, yet when I pulled on the knotted stern ends, that portion was floppy, even when the front was loaded.
Here was the culprit - broken internal rubber strands, which you can see as a reduced neck in the bungee diameter
This bungee has to be replaced.
Close inspection of the remaining bugee cords on both boats showed the rest is good.
Any time you see bungee outer covering frayed to the point that you can see rubber strands underneath, jack it up and put a new one underneath (you know, replace it).
But for little fuzzy frays in the bungee casing, just take a lighter to them - this slows down the fraying process.