And, relative to what
@Blue said, when you prompt your players for such things, or for things that might normally be established by the GM in more traditional play, they might not believe you're giving them that authority, or they might need a little time to come up with something like that on the spot, so be sure to give them that time (at first; they'll get faster with practice).
If, on the other hand, they clearly don't
want to be contributing in that particular way, you can let it slide.
Edit: Fixed a typo.