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the described symptom almost always point to an internal hardware issue or more particularly, possible suspects are:
• low or missing sensor supply voltage (least likely);
• cracked board, loose connection, cold solder sensor circuitry (likely);
• remote sensor of DVD player (most likely).
Still, there is a test to confirm if an IR remote is really working or not. With fresh batteries correctly installed, point the remote towards the lens of an active digital camera. For every press of any/all button(s) on the remote, there should be a bright light that would register on the LCD screen of the camera.
No light, no infrared transmission from the remote; remote most likely faulty.
On the other hand, if the remote is good, then consider a possible internal (sensor circuit) hardware concern in the unit itself or in some instances, a compatibility issue.
Please post back results of camera test.