Troop Ships

Save your game, quit, and then reload your save game. You should then be able to invade again... If the enemy invades one of your planets, you'll have to keep reloading to fix it again (assuming you are autosaving every turn).

Do they have troops on them?

you can't launch one without troops

Sure you can.
Two ways, but both have to be employed when built.

1) have the planet send all ships to a rally point, the ship will automatically go without any troops.

2) When the ship is built, right click on the ship icon the appears on the right side of your screen... I use the right click method on most of my ships and get annoyed when I use it on troop transports that get sent to a planet, then have to go back for the troops.

Aha!! My problem seems to be no troops on the ships. For some reason, occasionally when I launch a troop ship, it doesn't give me the slider bar to adjust how many troops to load. I didn't notice untill someone here suggested no troops on the ship. I launched troop ships from different planets and most of the time it gives me the opportunity to load troops and sometimes it doesn't. Hmmmm........
Thanks for the suggestions everyone.

forget to add the troop module maybe ?

Make sure your planet has enough population to fill the transports. If its a small planet and you build transports frequently, it can drain the planet pretty quickly.