Neo Geo CDZ problem...

I am having a problem getting specific games to load on my Japanese CDZ system. When I try to load a game, the system will make an attempt, but will go to the main system menu. I've tried this with all of my games, and all I have been able to figure out is that after checking the back of the discs, the games that would load (World Heroes Perfect, Ninja Combat)have a difference in the small writing around the center of the discs. These disc will have ADCD written on them, while the disc that won't load (KOF'98,KOF'96 Collection, and Samurai Spirits RPG)have NGCD written there. Can someone please help me figure out the problem?
Thank You

CD readers of any description can be a fickle thing. It may be that the optical pick up needs cleaning (if your lucky) or the focus and or servo tracking set up needs adjusting. Could also be a fault in that area.
Hope its not the latter, this a is tricky operation with special equipment required.
You could try using a CD optical assy cleaning disc, it has a little brush that clears dust off the lens, they have limited success but worth a try, if its a top loader try cleaning manually.
Use a swab with some iso to very gently clean the "birds eye"( its very fragile, dont push the lens hard you may damage the suspension). I use cotton buds, some others rekon they leave too much lint, imho if its a fresh bud the fibers are well compact and dont cause a problem.
Good luck.