In order for your Trackbacks to be sent out, you need to ensure that you have data in the field specified in the “Which field should be used for Trackbacks excerpt?” setting under Admin > Weblog Management.  For instance:

1. Say you have your weblog set to excerpt from the “Summary” field for Trackbacks.

2. You create a new entry, but don’t put any content in the “Summary” field.

3. You specify a Trackback URL and submit your entry.

Because you don’t have any data in the “Summary” field, ExpressionEngine cannot send a Trackback out; you need content in whatever field you have specified for the excerpt.  Generally, you should specify a field that you will always fill out for your “excerpt” field.