Yeah I get it. But you just can't quite dial it in, right? So what are you missing?
If you have had this guitar playing beautifully, than it probably just needs a couple tweaks (unless the neck has warped while in your possession....).
Hopefully it's not as bad as the slug player above, but here's what you do.. in order!
1. Check the neck- Look for any twists, humps or dips. If there is nothing evident, you need to dial in the relief (a.k.a. neck bow). There is an earlier post all about neck relief and truss rod adjustments here http://www.learn-guitarsetups.com/repair-blog/how-to-adjust-your-truss-rod. Give it a read and come back for the next step.
2. Now you have your neck relief set, with just a tad of relief, right? Now let's look at your string height at the saddles. Because I have no idea what kind of guitar, string gauge, tuning, or technique you possess- I'd first suggest to set your string height to the manufacturer's suggestion, and adjust from there. Give it a google search. Guitars with 2 post bridge adjustments (Tune-o-matics, Floating Tremolos, etc), it's as simple as raising or lowering each post until you are happy. If it buzzes, raise it until it stops. I'd suggest a little higher on the bass side since those strings need more room to play out without buzzing. Guitars with individual saddle height (strat, tele, etc), it's a little more involved then that- To correctly adjust string height, each string must measure off of and follow the fretboard radius- That may sound confusing, so I found a great video to demonstrate. There's a plethora of others out there as well worth checking out.
If you're still having trouble after these adjustments, consider bringing it into a pro. Or If it's never gotten a setup, that's exactly what is needed! Pickup our easy guide and have it covered in an afternoon or join us at one of our Precision Guitar Setup Classes!