AntPlusListener events not firing

In my Watch App i am trying to get an event to fire every time an AntPlus device data is updated.  Specifically every time power is updated from a power meter.

To do this I am trying to use BikePowerListener.

if I copy the code from the example in the documentation: 

https://developer.garmin.com/connect-iq/api-docs/Toybox/AntPlus/BikePowerListener.html

And remove the ListItem references as I can't see how to use them, everything initializes fine, but the OnCalculatedPowerUpdate event never fires.  Elsewhere in code I am acessing the data using the "getBikePowerData" function below, so the device is connected and working.  Why isn't the callback firing?

Here is my class:

using Toybox.AntPlus;
using Toybox.System as Sys;

class antplus_bpwr extends AntPlus.BikePowerListener
{
	hidden var _antplus_bpwr;
	hidden var _antplus_listener;
	hidden var _sample_count;
	hidden var _samples;
	hidden var _head;
	hidden var _max_samples_count = 12;	// 3 seconds at 4Hz..
	
	function initialize()
	{
		// Initialise BSC device:
		BikePowerListener.initialize();
    	Sys.println("BWPR Initialized.");
		_antplus_bpwr = new AntPlus.BikePower(null);
		_sample_count = 0;
		_head = 0;
		_samples = new [_max_samples_count];
	}
	
	// Class to store data.  Currently just power but could be other stuff too.  
	// Keep like this for consistency.
	class bpwr_data
	{
		var power;
		function initialize()
		{
			power = 0;
		}
	}
	
	// Function to return bike power data object.
	function getBikePowerData()
	{
		var ret = new bpwr_data();
    	var info = _antplus_bpwr.getCalculatedPower();
    	if (info!=null)
    	{
    		if (info.power > 0)
    		{
    			ret.power = info.power;
    		}
    	}
    	return ret;
	}
	
	// Returns device number.
	function getDeviceNumber()
	{
		return _antplus_bpwr.getDeviceState().deviceNumber;
	}
	
	// Callback on power update.
	function onCalculatedPowerUpdate(data)
	{
		Sys.println("Updating power");
		_samples[_head] = data.power;
		if ((_sample_count + 1) < _max_samples_count)
		{
			_sample_count++;
		}
		_head++;
		if (_head == _max_samples_count)
		{
			_head = 0;
		}
	}
	
	// get average power.
	function getAvgPower()
	{
		var ret = 0;
		if (_sample_count == 0) {return ret;}
		for (var i=0;i<_sample_count;i++)
		{
			ret+=_samples[i];
		}
		ret/=_sample_count;
		return ret;
	}
	
}