When you prepare & cook the required dishes, parts to help build the goal appliances will drop at random which are then stored in the collectables tab. Just make sure you are choosing the matching part needed to complete the appliance yoy are building.
You also need to make sure you are accepting the required parts for the appliances you're currently building. Don't forget to request parts needed which there is a tab located when you tap on an unbuilt appliances.
I hope this helps... Feel free to send me a friend request, my ID is SBDOVER72