If you're looking for a customer or vendor transactions in QuickBooks Online (QBO), the Advanced Search option is a good feature to help you find them.
Go to the magnifying glass icon at the top menu.
Click the Advanced Search link.
Customize the filters and click Search.
If you're still unable to find them using the said tool, there is a possibility that the transactions were modified or deleted. You can use the Audit Log to trace down all the activities and changes made within your account.
Go to the Gear icon.
Under Tools, choose Audit Log.
Click the Filter button.
Customize and filter which information you want to show in the list.