some disks play, some do not?

I cleaned all the disks. I took the Aiwa CD player apart. Some disks spin and play when rotated to the lens. Other disks, when rotated to the lens, start to spin for just a second, stop and then the disks rotate to the next disk. If I give the disk a slight push, it will spin and play. Sometimes the disk that wouldn't play the first time around, plays the next time it rotates to the lens ??? Any ideas? Thanks