Ah… Right… Are you perhaps using a bravo honeycomb throttle hardware? Or something similar?
Because I do, and when I keep the mode-selector button in the Vertical Speed-mode, it will refuse to follow a glide path. It’s like its holding the autopilot V-speed mode in zero, regardless of whatever I’m telling it software wise.
Whenever I turn the knob on the hardware to heading-mode instead, it follows the approach mode as it should.