Sometimes it's just wrong parameters (e.g. too low density, too low gravity, ...), but the 'jelly-like' behaviour may also help to keep the simulation stable (sometimes in games you can see the physics simulation explode, e.g. stuff being catapulted with near-infinite speed into space, putting some dampening into the simulation and allowing some 'slack' for resolving object intersections can help to avoid that, but also makes everything feel more jelly-like).