You will only receive a Registered Open Receipt if your recipient opens the email in a specific time frame: beginning after you initially received your Registered Receipt from the RMail you sent and ending 30 days after you have sent the email through RMail. It is helpful to know the difference between an Open Receipt and a Registered Open Receipt.
A sender will not receive an Open Receipt email for a recipient that already shows the status “Delivered and Opened” on the Registered Receipt email (like in the example below).
So check the Registered Receipt for the sent email in question to see if the Registered Receipt already reports the open as “Delivered and Opened." The RMail system will not send you a Registered Open Receipt because your Registered Receipt already shows proof that the recipient initially reported opening the RMail you sent them.