There may be two reasons for this:
 

1. Items that drop by chance


Some items do not drop every time when you collect profit/goods, but drop randomly, i.e. by chance. Sometimes it may be hard to collect these items as random drop is unpredictable, but this is not a game error. Try to collect profit/goods more often and you will definitely collect all the required items.

Items with a drop chance less than 100%:

  • Saplings — drop from the Summer Houses ("Forest Spirit" quest)
  • Snowboards — drop from the Shoe Shops ("Mountain Hotel" quest)
  • Rotators — drop when you gather Potato crops at the Farm ("Weather Station" quest)
  • Springs — drop from the Textile Factory when making Wool ("Weather Station" quest)
  • Items required for producing decor at the Academy of Wizardry:
    • Fiery Crystal — drops from the Bakery
    • Powder of 'Wisdom' Crystal — drops from the Museum
    • Pine Bark — drops from the Drugstore
    • Fancy Web — drops from the Villa
    • Merry Berry — drops from the Fruit Farm
    • Flower Dust — drops from the Flower Kiosk
    • Powder of Mithril Ore — drops from the Electronics Plant
    • Mandrake Root — drops from the Fruit Farm
    • Dreaming Net — drops from the Textile Factory
    • Sleepy Mushroom — drops from the Drugstore
    • Stardust — drops from the Japanese Castle

Important! Some event items may also be dropped at a random rate. If they do, the quest window will say "Do not drop every time" near the corresponding item.
 

2. Items with limited drop rate


Some items have limited drop rate per day:

  • Booklets — you can collect no more than 11 per day
  • Bistorts — you can collect no more than 7 per day