I played as a male protagonist and discovered that the Yarn Maker appeared in the Tool Shop exactly one day after shipping 50 wool. A note also appeared on the board in the square. I continued shipping wool (up to 100), and the Knitter never unlocked.
Then, I played as a female protagonist on my second save file. Even though I had maxed out Bill's friendship, neither item appeared. However, when I shipped 50 wool, both the Yarn Maker and the Knitter appeared in the tool shop the next day. Two notes appeared on the board in the square: one for each item.
(Interestingly, when the butter and cheese makers unlock, only a single note appears on the board in the square but it mentions both items. These items also unlock after a certain amount of milk is shipped-- somewhere between 50-100 milks total... I wasn't keeping track of that as thoroughly since milk is a lot easier to obtain!)
I guess I should note that I had 4 sheep in the barn, too.
Hopefully, this solves help a mystery for those of you who are curious and/or still playing this game in 2024 and later.
