Biscuits go soft once they get damp and lose all their crunch.
Tips
usage
发 before an adjective means 'to start to turn or feel': 发软 'to go soft', 发麻 'to go numb', 发冷 'to feel cold', 发红 'to turn red'. With the legs it is the weakness of fear or exhaustion: 腿发软 'one's legs give way'.