I have some Martin SP Phosphor Bronze 0.013 gauge strings on my acoustic guitar. The D and G strings (Strung normal EADGBE) have begun to wear at a few frets (3rd and 10th) and the winding on the guitar string is gone. All that remains is the center string. I can't figure out what is causing this. The strings are only weeks old and replaced every couple months. The frets don't appear to be ready for replacing. Any ideas as to what the cause of this problem is?