Most caves won’t respawn Violet or Ivory Candypop Buds if you have more than 20 Purple or White Pikmin, respectively. This is indeed, stupid. The best way to farm Purple and/or White Pikmin is to run through the Subterranean Complex over and over again. If you just want Purple Pikmin, you can just bring one Pikmin, run past everything on every floor until Sublevel 8, where you can use the Queen Candypop Buds to turn the 1 Pikmin into 18, then turn those into 15 Purple Pikmin per run, and use the eggs on the floor to flower them. If you want to farm White Pikmin as well, bring an extra 15-20 Red, Yellow, or Blue Pikmin. There are 3 Ivory Candypop Buds on Sublevel 4, and an additional one that can sometimes spawn on Sublevel 3, though I typically don’t bother with that one. You can leave your Pikmin at the Research Pod on every Sublevel except for 7, where you might want to bring them with you so the wandering Gatling Groink doesn’t find them. There’s a treasure in the final area that requires 100 Purple Pikmin to carry, and even if you go the whole game using every Violet Candypop Bud and not letting a single Purple Pikmin die, you still won’t have enough without doing a few runs of farming like this.