This will happen if the ladyfingers are soaked for too long. Excess liquid held in the sponge fingers will leak into the dessert as it sits in the fridge turning it watery. Soak the ladyfingers for a few seconds on each side (I usually count to two then flip them over and count to two again). If left to soak for too long then they may disintegrate when picked up as well as turning the dessert watery.