Minecraft wiki entry for Obsidian_farming, associated text: If you built the Nether portal too close to the generated one, it may bring you back to the original Overworld portal rather than creating a new one. The Overworld portal may then do the same thing and not generate a new portal. Conversely, if you built the second Nether portal more than 16 blocks from the first, the generated Nether portal may be more than 128 blocks from the original. For this reason, it may be necessary to adjust the location of the second portal in order to find the optimum distance.