afaik there are two options of why that is happening
per default seeds will be used for cooking. You need to toggle that off in the kitchen settings.
to get new seeds from your grown crop you need to brew alcohol from the plum helmets (and not use them for meals, but that should be off by default). So you should set up a brewer with the task of making alcohol from plum helmets.
correct me if I am wrong
It's working. On behalf of all my dwarves I say thank you! 😁
The number one cause of that happening is cooking the seeds. But if you are really sure that's not it, then it might be that you are using in some other job like milling/pressing, or some critter is eating them all, or it might be the bug that causes seeds to become unavailable once stored (but I'm pretty sure that has already been patched time ago).
What you can try is selecting a plump helmet spawn and make the camera follow it to see where it ends up going.
bug that causes seeds to become unavailable once stored (but I’m pretty sure that has already been patched time ago).
As of last year the bug was where keeping seeds in a bag meant that a dwarf would take the whole bag of seeds to plant one helmet. That would cause all the other seeds in the bag to be unavailable and would cancel all the plant orders. I was still setting up quantum stockpiles to deal with this one.
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