There are two possibilities why some orders may not appear in Gladly:
Only the 10 most recent orders of a Customer are pulled from BigCommerce to Gladly.
Gladly will only display orders in the Customer Profile if there is a corresponding profile in BigCommerce. This means if a Customer places an order (or an order was imported) and no Big Commerce profile exists with that Customer's email address, the order will not appear in Gladly.