HELP: (DESC BELOW)

I need help with my city to explain, last month I reached a peak of 400k population but now month later I have only 320k and I've lost 80,000 residents somehow out of nowhere. I have same population i did during January of this year! I don't want the problem is i have like 100 clinics and hospitals, hearses, and no space elevator..I have so much residental and a good economy? Whats the problem (XBOX and C:S1)