Okay, so, unfortuantely, 18 Alcremie forms are pretty much no longer available(all the forms that could be obtained through Ribbon and Star Sweets, which were through events that have passed), all the others are available at Hammerlocke (Get the Love, Flower, Clover, and Berry Sweets from battling Cafe Master Bernard, and the Strawberry Sweet is found by the Pokeball statue in the eastern part of town), Motostoke(All 5 are obtained through battling Cafe Master Dwight), and finally at Wyndon , again by defeating Cafe Master Richard(this also includes all 5). I'm 90% sure that the Sweet they give you, or if they give you one, is random, so you're just having bad luck I guess.
Anyway, it's almost impossible to get them all, as Ribbon and Star Sweets are no-longer available, the only way I'd think you could get one of those forms, would be through trading, and having someone attach one as a held item to their Pokémon, then again, I've never played SWSH, so I don't know if that's still how trading works, or if you can even have the Sweets as a held item, though you probably can. It was just made known to me by Polaris, that you actually can get a Star or Ribbon Sweet, as an extremely rare item from the watt trader in the DLC, so that's the only way to get some of those 18 forms.
Source: the pages linked in both the description and this answer, and comments.
Please correct me if I missed something.
