As I get higher up the neck (12th fret and up is when it starts getting noticable), the strings start getting flat...in other words, an open E is in tune, but at the octave it is a little flat. I had the intonation and truss rod and everything set properly a few months ago when I put on heavier strings.
What is causing this and how can I fix it. (I was thinking that it's just a matter of the strings being worn out but I'm not sure).
Thanks