I have had it 4 times now where I try to expose the star metal ore with fire orbs but the rock never breaks down. I made sure to hit it dead on with the fire orb, but nothing happens. And other ones I have no problem with, one shot with a fire orb and it breaks apart. I also tried keeping the flames going with grease orbs. I went through about 8 grease orbs on each of the nodes I had trouble with. Still nothing.
The first time this happened, I noticed that the star metal node was partially inside a rock. After wasting 8 grease orbs, I tried breaking the rock first and then hitting it with the orb. Once the rock was gone, I was able to break the star metal node with one fire orb. The other 3 times though, this was not the case - there was no rock partially merged with the star metal orb.