Four checks, in this order.
1. The date range
A transaction is filtered out by the period before anything else. The date used is the date the transaction cleared, which for a card payment can be several days after it was made.
A payment made on the thirty-first and cleared on the second belongs to the following month, and looking at the month it was made will not find it.
2. Which account it belongs to
Where several accounts are connected, the view may be filtered to one of them. A payment made on a card is not in the checking account.
Sources lists every connected account and what each reports.
3. Whether it is waiting to be categorised
Transactions arrive before they are assigned to a category. Until then they appear in the review queue rather than under a heading — see uncategorised transactions.
This is the usual explanation when a total is right and a category looks short.
4. When the connection last brought data in
Banks deliver transactions on their own schedule, and a recent transaction may not have been sent yet.
Sources shows the last time each connection received data. Where that date is older than expected, see a connection stopped bringing data in.
Cash and cheques
Money that did not move through a connected account has no source to arrive from and is entered by hand.
If it is still not there
Tell us the amount, the date, and the account it should be on — see getting help.