This is how I tighten the cog/lockring, worked well for me. Do this and also check your threads for damage. You must have a good quality lockring wrench for this. I use a Hozan myself.
1) with wheel off thread on cog until hand tight. Thread lockring on but not all the way.
2) mount wheel on bike with correct chain tension. Then pedal real hard to create the torque necessary to fully tighten the cog
3) with the cog fully tightened on the bike, tighten the lockring on until handtight, then take a lockring wrench and tighten all the way. Use the whole length of the wrench for leverage.