I was hunting a Steel Uragaan when, due to my Capture Guru, I knew he was ready for a capture. I placed the trap as close as I could to him since he had rolled into the lava. Upon trapping him, I threw Tranq Bombs at. Repeatedly. To no effect. What went wrong?
So, I did some testing and I believe that I've found the answer. After attempting to trap an Agnaktor, an Uragaan, and a Steel Uragaan, I've reason to believe that if the monster is trapped on the edge of the lava (i.e. you place the trap right at the border between lava and walkable land), then the monster cannot be captured. While you may still be able to damage portions that are close enough over land (e.g. Agnaktor's beak or Uragaan's chin), you will not be able to capture the monster.