Run a clean comparison
Test one known-good item on both browsers, then repeat with the failing item. Note whether failure occurs at sign-in, when the player loads or after playback begins. If available, record the browser, version, operating system and the reported media format.
Use that result
If both browsers load the site but only one plays the exact item, send the publisher the device and format details. If neither opens the same account, check the account or source retrieval before trying another browser. Keep account cookies and network logs private.