No data appears in the Email Performance Report (configured for a specific time frame) even though emails have been sent to the leads.
The reason why the leads who were sent the email do not appear in the email performance report configured for a specific time frame is because the Sent Date filter is based on the first date that the email was sent to a given lead. Email Performance Peports only show data for the first time an email was sent. If you send the same email multiple times to the same leads, reports intended for later sends will not include leads who first received the email outside of the report's time frame.
Apart from that, it is also worth to check if the email performance report has any additional smart list filters that the leads may not qualify for.