We are trying to log sends in a Send Log DE in Sandbox SFMC BU, but the data is not getting populated.
Does Send Logging works in Sandbox?
Send Logging works in Sandbox accounts. You should verify that there aren't any other active SendLog Data Extensions in the Business Unit, and/or that your account is not setup to log everything the the Enterprise account SendLog. As well as, that the Data Extension you are using is built from the SendLog Data Extension template.