You probably were wearing the tough guy sunglasses or some other + fortitude buff when you purchased it. Then later you changed to different glasses or lost your fortitude buff, which disables the perk til you either spend another point in fortitude, put back on the fortitude eyewear, or use the food that gives a fortitude buff (i think there is one, but i can't remember which).
Personally, i keep a chest by my farm with seed mats and the tough guy glasses. Once i have 5 of whichever food i am still crafting seeds for, i put on the fort glasses, craft my seeds, then put them back in my farm box.