I would agree it may have gotten too warm. It looks like your heat lamp is over one side, but they still need room to get away from the heat. Do you have a small thermometer you can put down in the box? The solid sides can tend to keep more heat in than it appears. You may need to raise the lamp.
And then some chicks just fail to thrive, it happens, so if the others seem ok, it could just be coincidence.
And then some chicks just fail to thrive, it happens, so if the others seem ok, it could just be coincidence.