Why have my 3 mystery snails died in the past 2 days!

I have a 55gal freshwater aquarium that has been set up for 6 months now. I have platys, guppies, a rainbow, 2 gouramis and a pleco. I also had 2 black mystery snails and a blue mystery. Two days ago one of the black snails died and now today the other two have died, what is going wrong with my tank? Am I doing something wrong? Three days ago one of my guppies had fry so I am assuming the water quality is good, especially as I have many live plants.