Why are customers sometimes unable to complete orders on my platform?

My customers are weekly based. Some weeks they are unable to complete thier order as normal. They won’t be offered payment options other than shopify pay. They aren’t allowed to complete their order because it says order can’t be shipped or picked up. These are the same orders, same customers every week. Yet some weeks my sales drop over 50% because customers are unable to order. Its frustrating to say the least when you run an entire business from a platform that just decides to randomly not work. Why is this happening? It’s impossible to receive help from shopify themselves, so hopefully the community can help.