My Martin that I just bought (DXK2) has a set up problem, I believe. The first and second (high e and b) strings ONLY do not ring out on the 12th and 13th frets. All other frets on all other strings work out. What could this be? The guitar came with light gauge strings and when I changed then I put light gauged strings. It worked before but now...its now ringing out (very sharp staccato note). Could this be a nut, bridge, neck, or surface warping problem? I keep my guitar in its hard shell case ALWAYS unless I am playing it and I store it in my dorm right by my desk and my room is always at 70 degrees F. Any guesses?
well i would have said that it's definitely a problem with the frets... but it's odd that it only started doing it after you restrung the guitar. i'm still led to believe you've got a problem with your frets though. take it to a luthier or shop and see what they have to say about it.

honestly i can't be comfortable making any definite recommendations on what you should do without seeing the guitar though. take it in to a shop is my advice.
It's a problem that is very uncommen with steel string guitars but very common with nylon string guitars. Basically, the strong and weak spots in your guitar top are not in the right spots. On a solid top guitar you would need to put a weight on your guitar top and move it around while you play. When you get the weight over the right spot on the guitar, you will find that the sustain comes back to the dead frets. When you find these sponts you would then reach into the guitar through the soundhole and sand off a little wood from the top where the weight made a difference. You would keep sanding untill the problem stopped. On a Martin X series guitar you cant just sand off a portion of the top so I guess I would try and take it back. It is down to a defect but I don't know if it'll be seen that way in the long run. See what you can do.
Not taking any online orders.
Ah. I see. I will have my guitar checked out by an authorized Martin dealership. Do y'all think that I can get my money back from Martin? or just another DXK2?
^I don't know what they will give you. Don't know if they will even fix it or recognize that there is a problem. If you can get your money back than I would. Then get something that is made with real laminated back and sides and a solid top. Not only will a solid top guitar sound better, but it'll hold up better, last longer, and be easier to fix when the time comes.
Not taking any online orders.