For clarification sake, does this mean they are not on your farm (I.e. placed outside your farm) or was that a typo? From a quick search I see there was an old bug where they needed to be placed outside,
on your farm, for the recipe to trigger, so I thought I'd ask, have you tried that?
Others suggested collecting them all and putting side by side to 100% confirm all were obtained and one wasn't destroyed by weeds or hit by lightning or something in the meantime (the player in question lost one without realizing):
www.reddit.com
Some had this problem with old games where one or more rarecrows were obtained in previous versions, they had to obtain those again to get it to register together. Also, some who had this problem suggested re-buying one or each rarecrow, keeping in inventory, and then placing it/them again, on the farm (at the same time), then waiting the requisite days to get it to trigger. The number of days that pass before it triggers seem to vary for players. There is a 90.05% chance each day if it happening. Bad luck/RNG could push it past 4 days, though the odds are rather low for this, it is
technically possible.
www.reddit.com
People say they shouldn't have to be placed to register (based on code reference) but for others they did. Some who got them at different times (having bombed prior ones) had issues with them all being registered together without being obtained together, though it is marked as solved:
Not sure if any of that is helpful but thought I'd ask for clarification that might help the next helper.
