why i can't get the material from floder except the first"play"?

oh,guys, sorry my bad english, but i want to know, wheni wanna change the material when the actor has created. but i alawys can’t get the material from the engine, except the firstime. except i restart the editor, and it will happen again. Normal->can’t get->breakpoint… please,help me , i’m the newer of ue4.