Reicast lose response when loading a game!

hey friends! I have download the retroarch 1.5 X64 for windows, and I have put the dc_boot and dc_flash in the right place: system/dc. when I load the reicast core and a DC game, it will appear the DC LOGO, and then it’s setting time. when I have done this, the window always lose response, so I have to force close the retroarch window. what’s THE problem? NEED SOME HELP! THANKS!