If your popcorn balls aren't holding together, it may be due to the marshmallows not being fully melted. Make sure to stir them until they're completely smooth before adding the popcorn to the mixture. Additionally, there may be too much popcorn in the mix. Add more melted marshmallows if this is the case, or only add some of the popcorn at first until you get a stickiness that works for you to shape the popcorn balls.