Why does my DC no longer play backups???

[gr8dane]

I have a DC I recently just bought used. It would play backups at first no problem. Now it gets to the spinning cd screen and that's it. I can put in an original game and it will work. This one was made in August of 99. I also have another DC console made in September of 2000, but I do not have any problems w/ it. PLZ help , one of these consoles is for my sister's b-day.