The IR (infra red diode) could be low emission.
Remote faults always come down to just a few things...
1/ Tarnished battery contacts.
2/ The IR diode.
3/ The ceramic resonator can fail (often a leg snaps off) although this causes a totally non functional remote.
4/ Dry joints.
5/ The PCB top surface becomes 'gooey' from plasticiser leaking from the conductive rubber keypad.
Does the Quad use standard Philips code ? If so then try a universal remote and whizz through a few Philips and Marantz players to see if any work the Quad. That would prove whether the fault was with the remote.
If you have a scope then its possible to look at the IR receiver output in the player. It should be a nice 5 volt train of data when a key is pressed. Receivers sometimes fail but are cheap and easily replaced.
Remote faults always come down to just a few things...
1/ Tarnished battery contacts.
2/ The IR diode.
3/ The ceramic resonator can fail (often a leg snaps off) although this causes a totally non functional remote.
4/ Dry joints.
5/ The PCB top surface becomes 'gooey' from plasticiser leaking from the conductive rubber keypad.
Does the Quad use standard Philips code ? If so then try a universal remote and whizz through a few Philips and Marantz players to see if any work the Quad. That would prove whether the fault was with the remote.
If you have a scope then its possible to look at the IR receiver output in the player. It should be a nice 5 volt train of data when a key is pressed. Receivers sometimes fail but are cheap and easily replaced.
It is infrared, but it is still a "visual" signal.
Most of my remotes now have a bare LED sticking out the end, but some have a decorative cap over the LED, which can accumulate dust and dirt.
If your receiving unit is like mine, the sensor is behind the panel "glass". The glass is usually colored plastic, and depending on the environment, it can accumulate a layer of dirt and dust too. SO take it apart, and expose the inner side of the window and clean it, and clean the lens on the sensor.
Most sensors are small metal cubes and so hard to knock out of alignment, but check yours to see if possibly the sensor element has got crooked.
It has to modulate the IR LED, but bright ambient light may interfere with the sensor.
In the remote, what Mooly said. Rubber contacts do wear out or get dirty, but if it works consistently up close, then I doubt that is the issue.
Most video cameras pick up infrared. If you shine the remote into the lens, it should be easily seen as red light. Does it look strong or weak?
Most of my remotes now have a bare LED sticking out the end, but some have a decorative cap over the LED, which can accumulate dust and dirt.
If your receiving unit is like mine, the sensor is behind the panel "glass". The glass is usually colored plastic, and depending on the environment, it can accumulate a layer of dirt and dust too. SO take it apart, and expose the inner side of the window and clean it, and clean the lens on the sensor.
Most sensors are small metal cubes and so hard to knock out of alignment, but check yours to see if possibly the sensor element has got crooked.
It has to modulate the IR LED, but bright ambient light may interfere with the sensor.
In the remote, what Mooly said. Rubber contacts do wear out or get dirty, but if it works consistently up close, then I doubt that is the issue.
Most video cameras pick up infrared. If you shine the remote into the lens, it should be easily seen as red light. Does it look strong or weak?
I dont know if it uses Philips code and my universal remote is very old and doesnt include cd players.
I will look at it with the tablet next time I visit its owner, Its not mine but a friends.
Since you say the batteries are fresh, the next thing to check is the battery contacts. I've had more trouble with those than with batteries, I swear. 
If you can find one of those rare pens (biro) with gritty rubber eraser on the end, they are excellent at cleaning contacts. More aggressive than the pencil rubber, and will remove most light corrosion and oxidation.
If you can find one of those rare pens (biro) with gritty rubber eraser on the end, they are excellent at cleaning contacts. More aggressive than the pencil rubber, and will remove most light corrosion and oxidation.
If it is only some buttons, and the buttons are silicon, try taking it apart and cleaning the board (damp rag, perhaps some alcohol) and the rear side of the buttons (I just use liquid dish soap). There is a conductive pad on the back of the buttons and this can get damaged, but usually it is just the greasy leachant from the button material (silicone) that tends to make the remote a bit fiddly.
The contacts on the board are usually gold plated and don't need some sort of abrasive cleaner. The material on the back of the buttons can be renewed, although I would try cleaning the board and button pad of "grease" as that might be all that is wrong.
https://www.amazon.com/Keypad-Restore-Conductivity-Carbon-Copper/dp/B0026PRMVM
The contacts on the board are usually gold plated and don't need some sort of abrasive cleaner. The material on the back of the buttons can be renewed, although I would try cleaning the board and button pad of "grease" as that might be all that is wrong.
https://www.amazon.com/Keypad-Restore-Conductivity-Carbon-Copper/dp/B0026PRMVM