Why don’t the volume buttons on my bluetooth headphones and speakers work on iPhone 6s?

About 3 weeks ago, the volume buttons on my Bluetooth headphones and speakers stopped working on my iPhone 6s? When you press the buttons on them, the volume slider symbol should appear on the screen. It stopped appearing and I can't adjust the volume via Bluetooth devices. I can adjust the volume with the two volume buttons on the side of the phone. Any suggestions?