An invoice will only display a change order if it was created after the change order has been fully approved.
This behavior is not retroactive. Even if your change order is fully approved afterwards and you kept your invoice as a draft, the change order will still not be included.
I accidentally created an invoice before a change order was fully approved. What can I do to fix this and see the change order?
If your invoice requires the change order, then your only option is to delete the invoice and recreate it. Depending on your access rights to the project, deleting the invoice may require assistance from your Projectmates site administrator.