I'm learning Jbeam and I'm making a few objects for fun and also to learn . My first object: It's basically a long cube on the top that supports a rope with a cube attached to it. (Rope mesh fixed in the 4th screenshot) Don't be too harsh on me xD this is my first object, and I have some nice ideas for new objects that will be a lot of fun in this game.
Can you paste the flexbody section for that rope? I'm guessing you didn't assign the mesh to both the swinging cube nodes and the bigger cube's nodes. As simple as this may be, it's really good you're putting the effort in to learn jbeams. Good work so far.