Is this on all sender machines at your end?
Does the sender allow read receipts to be received?
Does your ISP/mail server allow read receipts (some don't)?

Have the sender make sure that they not only enable the option to send
read receipt requests but also enables the option to ACCEPT read
receipts.

How do you know the customer is sending back the *requested* read
receipt?

Maybe you use anti-spam software or a mail server that strips out the
Disposition-To header from your outbound e-mails (but then the
customer wouldn't get requested to send back a read receipt).  Or you
use anti-spam software or a mail server that blocks inbound e-mails
that are read receipts.

Check if the sender that expects back the read receipts has any rules
that move or delete the e-mails that are read receipts.  Check the
Junk filter if you have junk filtering enabled in whatever is the
UNIDENTIFIED version of Outlook used by the sender.