#1
I got a brand new Epiphone non-electric acoustic at the end of November. Last week, the high E started playing the same notes between the 11th and 15th fret and they sound "dull" (I don't know how to describe it other than the note doesn't play on some of those in-between frets). I tuned it up but haven't changed the strings. Is this a common problem? What exactly should I do? *I have a Guitar Center nearby*
#3
It sounds like the 15th fret is too high. Since you've only had it a couple of months, Guitar Center might consider exchanging it, if it's in good shape otherwise. If they won't, then you'll have to take it to a repair guy to get the frets dressed.

Edit:
Then again, a truss rod adjustment might fix most of the problem.