If you're finding that a CN22 isn't printing for a specific order the first thing to check are the order totals for that order. CN22's have to declare an items value and the order total value so not all values are accepted.
If the order total or line totals for any order lines is 0 or a negative value then the CN22 will not generate. In these situations please alter your order value/product values so they are valid for the CN22.
Another thing to watch out for is discounts with low value orders - to provide the total value of the goods we take the order subtotal (which is before shipping costs, discount and tax) and deduct the discount from it to try get the value of the items.
In cases where there are high shipping costs and the discount is actually for the shipping there may be issues with CN22 generation. If you're struggling with this scenario please contact support for further advice.