Bridesmaids behaving badly are often assumed to be jealous of the bride, or out for sabotage.
If you're the maid of honor: Ask the bridesmaid if there's something going on in her life that's preventing her from participating in the planning. Empathy comes first: she might have a medical concern, family problem, or a money or school issue that's getting her down. If so, the bride will be relieved that it's not a matter of her "not caring," and you can step in to offer support.
If you're the bridesmaid: If you don't have a significant life dilemma slowing you down, it's time to check the green-eyed monster at the door. The bride will find out and she'll be upset that you betrayed her trust and took up a spot in her bridal party, when someone else could have been nominated. She'll think (perhaps justly so) that you just want attention and control, which could put your relationship on the rocks. If you value your friendship, be a better team player: respond to those messages and get payments in on time. If you don't think you can handle this position, then step out of the bridal party before it's too late.