Why are the form fields blank after I send the document, even though I have filled them in and can see the completed form before I send?

This can depend on a Sending Setting called "When an envelope is sent, write the initial value of the field for all recipients". If values are added prior to Sending an envelope those values aren't technically written to the envelope/document until the Recipient responsible for those fields has completed their portion of the action, unless of course you enable that setting I mentioned, which writes the values immediately after sending. You would need to speak to your DocuSign Admin to have the setting enabled in your DocuSign Account.