I'll look into it, it seems weird as I have won this level many times. Maybe a bug snuck into my level code.
Will report back asap.
Edit:
Just now I played hide and seek twice and won twice. I think the only time it is unwinnable is when the asteroid of the first invasion force has already planted a defense tree by the time you arrive.
You do know there are friendly seedlings to disover in this level?
Also, I assume you mean level 18, not 17?
Mh, I played 7 times, every time the enemy had planted a defense tree, and after I had conquered that asteroid only few of my seedlings were left.
Then I went to the asteroid w/ my friendly seedlings—sure I know about them (having played the game several times from beginning!) and fondly remember the times when they were super seedlings ;)—and chased them to my bigger asteroid, but then the enemy (sometimes several enemies) already furiously attacked w/ their many and much stronger seedlings so I had no chance <sob>
Spoilered for those that don't want to know it. Use only in extreme circumstances as this will mess up achievments and stuff. It causes the game to belive that you won the level even if you are just about to lose.
Try using devmode to bypass it. Remember devmode? Press Ctrl-D, then tilde (either ` , next to the 1 key, or ~) , then type Quit(true) The capital Q is important. In the next level press tilde again, then Ctrl-D. More devmode info here. (http://www.dyson-game.com/smf/index.php?topic=882.0)