If you notice that you are missing a transaction that should have been imported, the first thing to do is copy/paste the transaction hash into the search function, like this:
If this doesn't return any results, open up the search function again and select "show deleted transactions" like this:
If this search finds the transaction, you can undelete it to restore it to your account.
If this search still doesn't find the transaction, you can try reimporting the wallet, but keep in mind that reimporting can sometimes overwrite past edits. Another option is to add the transaction manually using the "create" button in the top right corner of the transactions page.