I was playing my Les Paul today and I noticed when i play the upper frets on the higher strings some of the frets dont make sound. Like they have sound but dont change to the next pitch. It was fine untill i decided to try a higher gauge string set. Any advise to help me out?

Thanks guys
Well yes but in the upper frets like 14th is the last note playable. Any other i play sounds muted or the same note as the 14th.