We need the specific model number, but it’s quite possible that two of the devices have different firmware levels. Specifically for the GE switches that came out in the last 18 months or so, the original release did not have the firmware updates for double tap. So they just don’t support it. And there isn’t any way to update them on your own – – you usually have to contact GE/Jasco support assuming they are still under warranty.
You can ask more questions in the author thread for that DTH and someone there may be able to help you figure out specifically what’s going on.