Refurbished iPod Classic 7th Gen, Compatibility Issues

I have just purchased a refurbished Classic 7th Gen, which is a direct replacement for for my 2013 version. Unlike that unit, which happily responded to commands from my 2 Sony Headphone sets, in-line remote controls, this will not. Audio is fine, but no pause,skip or volume control. I sent one back, thinking it was faulty,but the replacement is just the same. The company who refurbish them, suggest my headphones may not be compatible with the "newer" version.

Does anyone have any idea why?

Thankyou