The only way to get the key is to have them all howling, as you say, at once...and by themselves, too, with no finger-pushing from you...and nope, no combinations or special sequence is needed. But you've tried that, of course. What I've noticed about this puzzle that's a little frustrating is that it's very difficult to see when a creature is howling on its own 'cause my finger is directly over the creature, thereby blocking my view of it. It takes a few seconds to get one howling, though, so I just go by that...lifting my finger up for a second to see if it's howling on it's own. But the thing is that eventually a howling creature will stop howling...unless you have all seven going at one time. So my guess, Kencho, is that at least one creature has stopped howling before you've gotten to the other six.
Or...it is truly bugged for you, in which case I would encourage you to redownload and reinstall the game to see if that clears up the issue.