Please excuse me if I am misunderstanding your question, but here's how it works for me. Before I stand on the scale, I need to tap on the surface with foot until scale "wakes up" and displays 0.0, then step on to weigh.
Sorry if this is too fundamental but thought it might be worth a shot. Hope you get it figured out.
Has your problem was ever resolved? I am having exactly the same issue here with newly purchased scale.
After hours of trying i have contacted Garmin Support and they have told me that they will investigate it, without giving any time frame.