Old-school style beach plum jelly is made without any added pectin. When gathering them in the wild make sure to include some under-ripe berries. A long simmer makes for a good strong set. It's a sticky jelly with a good puckery punch.
Sometimes, things don't always go as planned in the kitchen. You can't always trust your equipment to work without failure. Here's a simple, classic method for telling when your jelly is set - without relying on a candy thermometer.