Default [Q] Long click on headset button no longer works

Long clicking the button on my headset no longer activates S Voice when in sleep mode. I've tried everything I can to solve the issue, but no joy. I bought a replacement headset in case that was the issue, but still the same. I've installed the Headset Button Controller app and tried the setting 'Prevent sleep mode', still the same. When I use the debug mode in HBC advanced settings, it detects all button presses apart from long click. It did work, but then just suddenly stopped. The phone is only about a month old, but I don't want to have to try and get a replacement. I'm just hoping that there's people far cleverer than me that might have some idea I can try and to fix this. Can anyone suggest anything? Thanks.