How to attach mounted flat-screen TV to the wall so that it lines up perfectly?
The adjustment is usually under on the wall part of the bracket. The bracket needs to be plumb in both planes (kind of left and right and then forward and backward). 

The better brackets are bolted back to the wall and then micro adjusted (via small adjustment grub screws) in all planes until absolutely spot on. Then assuming no play in the arm, all should be well. 

Have seen a few that are very difficult to get right in all planes though. I.e, TV looks good pushed back but wrong when pulled out etc. 

Using serrated spring washers helps hold the bracket firm if the problem is caused by bolt heads slipping in bracket slots.