Why is Apple Pay not consistently working on my online store?

Hello,

I have enabled Apple Pay on my Shopify store. I tried to purchase a product however it kept declining. I knew it wasn’t the card as I use the same card to purchase things with Apple Pay from all over. I turned Apple Pay off and then back on and I managed to successful puchase a product from my store. However I have just gone to try it agin before posting on social media that we now accept Apple Pay and it once agin doesn’t work. Can any one help?