Apple Watch 5 double click for Apple Pay does not work for user

Here’s a weird one. My wife cannot get the double click facility to pay using her Apple Watch 5. When double clicked it first brings up the wallet showing all cards in the wallet, then when the actual credit card is tapped with her finger the credit card comes up separately and when double clicked it just keeps saying “double click to pay”, very occasionally after many clicks it will finally work. Here’s the weird bit, when I double click her watch, whether she or I am wearing it, then it works immediately every time. Although I don’t think the double click feature uses skin conductance, it may be significant that her fingers are less conductive than mine and she has more difficulty with screen touch or sweep functions compared to me, and her finger print touch recognition can be set up but then never works on her devices.