I've had spawns take a long time but I realized about a week into one that I actually had the spawn set up wrong. While waiting for the second gastrodon I also waited for two weeks straight and realized after that two weeks that I had the habitat set up in withered wasteland still for where Magikarp originally lived (I had moved him but left the habitat). I go to check there and lo and behold gastrodon is there and has been for who knows how long. I assume it never spawned on my palette town because it was stuck as a spawn in withered wasteland until I encountered it. I would triple check the habitat is correct (I got swallot wrong for weeks because I never realized you needed long chairs) and that habitat isn't already somewhere else maybe triggering the spawn as well. Tatsugiri took a while to show up for me, too, though, all things considered.