In simple terms you have a flame that is hot enough but not enough heat. Confusing right J
The problem is while your flame is hot enough (degrees C or F), you are not generating enough heat (BTU or Joules in metric) to bring the mass of metal up to the melting point. You can address this in many ways, different torch, multiple torches or keep the heat that you generate from being drawn off by convection or conduction. The crucible and fire bricks are a good start, but there is no way a plumbers torch is going to produce enough BTU to melt the metal in the crucible, you might need 2 or 3 of that kind of torch going at the same time to beat the heat losses from your setup.