Terraria – Can’t Find Jungle Temple in Terraria 1.2

You aren't doing it wrong necessarily, but you do have some of the details wrong. For starters, the temple doesn't spawn in when you defeat Plantera, it is always there, it is just that you can open the temple without the key that Plantera drops, or without the Picksaw or higher tier pickaxe. As to finding it, it is best if you just start strip-mining the entire jungle and checking your map every once in a while to see where you haven't gone yet where there is an underground jungle. Asides from that there really isn't a trick to it. Just some brute-force mining and there you have it.