Hi,
when you select the mission in the sending window for fleets first and afterwards add all other necessary things the text for the mission is shown but the mission is NOT selected.
Example:
http://prntscr.com/kngqp8Here I selected first the transport mission and then I added the 1 queen. As you can see the text for the transport mission is shown. But the mission is not selected.
This works permanently if you remove the units again, select another mission and add units again. The mission text shows but the mission is not selected and the send button is not available.