The recipe is actually attached to the flowers themselves if you sell/craft them and reduce the amount to 0 the recipe will be removed.
So to retrain the recipe just keep one of each in your inventory :)
Oh, that's good to know that the recipe doesn't get removed, I must've fixed it somewhere along the way and forgot about it.
Hmm, I'm just wondering what else it could be.
Normally by picking up the flowers from the maps that should add to the recipes found.
If you're still having problems, do send your save file over on https://www.file.io/
and I'll take a look at it.
Alright, I've figured it out.
There's no Ring Flower to pick up anywhere. ( I must've missed this when I edited some Shell Flowers in).
I'll make a quick fix to include some!
I've added some in Green Valley and the Entrance of the Infested Cave.
And while I was at it included some more flowers for those who want to 100% the recipe/crafting items.