Default [Q] S Voice won't activate from headset

Until yesterday, I was able to activate S Voice using my S4 headset when the device was locked. Now for some reason it will only work if I first unlock the phone first. The music controls still work when locked, but long pressing on the headset button no longer does anything. It's really frustrating cos I don't think I've changed anything, and it's a feature I use a lot.

Does anyone have any ideas how to fix this? I'd really appreciate any help with this.