The most common cause for this to happen is that there is a placeholder that needs to be replaced. As a security mechanism to prevent unedited templates from being sent to translation, we won't allow the message to move until all placeholders are replaced.
This is somewhat of a common situation if you are using templates.
We consider a excerpt of text to be a placeholder so long as it is surrounded by square or curly brackets. This means that if your text is using these brackets for any other reason, you may face the restriction and need to replace them!