Welcome to the boards
Mandarins have a very specific diet and for that reason are a more difficult species to keep. Some people have reported success with mandarins eating frozen food, but the majority do not eat frozen at all. Rather they eat the sand bed microfauna (pods etc) that live in the substrate.
How long has your tank been up and running?
How much LR do you have?
What size tank?
What are you using for substrate? Sand, crushed coral, etc?
I hate to be the bearer of bad news, but if your tank is newer (less then a year) does not have at least 100lbs of live rock, with a sand bed, it is likely that your mandarin is going to starve to death, and sounds like it already is.