Check these in order: is it a work or school account in Google Workspace? Has the administrator allowed read receipts, and for which recipients? Did the recipient refuse, or does their program ignore receipt requests? Many personal mailboxes and phone apps never send them.
A tracker such as HawkOpen also works in personal Gmail, but HawkOpen tracks only recipients whose consent to tracking is recorded for your company. It shows image loads and clicks with a category and rule code, never proof of reading, and every tracked email tells the recipient it is tracked.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.