Most games were written for the A500, the A1200 isn't 100% backward compatible. You have to do some voodoo to get an A1200 to play nice, (I'm sure others will go into greater detail).
Back when I had my A1200, most of my games were either whdload-ed, or I just used WinUAE because old floppies were annoying and unreliable.
http://www.whdload.de/
