Just to confirm, you are tightening the lockring by turning it counter-clockwise (opposite of normal) right? Lockrings are reverse threaded. If the spacer is big enough that it pushes the cog far enough out that the lockring can't grab enough threads then it's possible the spacer is your problem. If you post a picture of the hub with the cog/spacer/lockring and also without the lockring we might be able to help better.