I recently made a purchase, but the transaction is not visible in the list of transactions, and the balance has not decreased. Why?

Normally your transactions will appear as reservations directly in the app and the amount that you spent will be drawn from your balance immediately. After a few days when the transaction is ready it changes status to Completed. If you use your card in an offline terminal (for instance during a flight) then the transaction will not be visible, and the balance will not be affected until after a few days when it's fully done and has the status Completed.