Did you get this SP used (most likely so I will continue)?
Try another advance game, if the same effect occurs I bet it's the metal lines thingys that read Advance games is wore out, but the ones reading GB games is still working. Happened to the third SP my dad brought home from the garbage. I personally couldn't figure out how to fix it at the time, but the guys I knew down at Microplay taught me how to fix it. You just got to replace the metal thingys in the SP with ones from another one.