You may have had the same issue we had. We switched to 1 page checkout, then had to revert, as functions were enabled on our POS stores, and we had not known that this was pushed live a few weeks before we made the change. We reverted that same day, but our customer service department reported that customers were having issues paying for orders for a few days afterwards. Eventually shopify “resynced” our checkout and everything has been fine since. Maybe your customers are having the same problems, but just not telling you.