Despite your concerns, did you all have a good time? I don't think you were 'cheating', I think you were playing the game as Adam and Sage intended! Iserith gives solid advice, but I thought of a few extra things to add...
When you say the players were unlucky, and you had to keep coming up with as hard a move as you'd like, I have a list of all my Principles and Agenda, all my Dungeon Moves, all my Standard GM Moves, and all the grim portents listed in front of me. These are your 'crib sheet'. I also note with a different coloured pen which options target specific player flags. Note that deal damage
is just one of about 2 dozen options!
Think of a golden opportunity (a player rolls a <7) as your
time (GM's are players too!) in the spotlight. What do you really want to introduce into the fiction? What excites you about the protagonists or situation? If you can't instigate that into the scene right now, how can you 'set it up' to introduce on the next failed roll?
If you are still 'stuck', rather than just go to deal damage as your move, I ALWAYS go with:
Tell them the requirements or consequences and ask
This move is particularly good when they've done something that's not a move, or failed a move. They can do it, sure, but they'll have to meet the requirements.
So you offer the players a hard decision. They can still do what they intended (sort of) but there is another, even more impending doom just waiting in the wings. This purposefully sets up a 'moves snowball' and allows the moves to flow from the fiction, which is the most satisfying integration of the story and the resultant die rolls.
In regards to balance, your role is to stick to the agenda, which purposefully does not include being 'balanced'. You always say what the rules and honesty demand though. The players need to know this, tell them that your job is to be a fan of the characters and fill their lives with adventure, and thus the player's will have a great time. DW isn't a tactical skirmish game balanced to the nth degree, its first and foremost a story
game. Fiction first, always.
Hope that helps?