Unfortunately starvation seems most likely, but you can't rule out natural casualties either. I raised my 3 toads from tadpoles and they're about 4 months old now. The amount they ate at that stage was astonishing, there's really no overfeeding them when they're that young imo. I started feeding as much as they could eat once a day and then I switched to feeding slightly lesser amounts 2 times a day to ensure the flies didn't have time to groom the powder off. Were you using a multivitamin?