One possible reason could be that the “next” and “previous” functions are overridden by another application installed on the phone.

Some applications enable users to customize the functionality of the in-line control on a wired headset. However, these applications may also override the remote control profile for Bluetooth headphones (AVRCP), which disables the "next” and “previous” functions.

To solve this, uninstall the application that is interfering with the Bluetooth controls. Alternatively, use the user interface on the phone to control playback.