Were the players close at all? Was it a case of the players spouting off synonyms for "balance" for an hour, and just not hitting on the right one? Or did they just not get it?
I think your hint about the word moderation was the way to go. Let them know they're getting close.
Very specific word puzzles like this can be tricky. I had a similar one once where the answer I gave was "emptiness", but the DM was looking for "nothing". He just told me that wasn't the answer, so I figured I got the riddle wrong, and went in a different, completely incorrect direction. If he had given a clue saying "almost, but not quite", I probably would have gotten it. Instead we sat and agonized over this riddle for much longer than was fun.
I think your hint about the word moderation was the way to go. Let them know they're getting close.
Very specific word puzzles like this can be tricky. I had a similar one once where the answer I gave was "emptiness", but the DM was looking for "nothing". He just told me that wasn't the answer, so I figured I got the riddle wrong, and went in a different, completely incorrect direction. If he had given a clue saying "almost, but not quite", I probably would have gotten it. Instead we sat and agonized over this riddle for much longer than was fun.