Dead fret problem

I have a little problem with my acoustic. On the high E string (the skinniest one), the 14th and 15th frets sound exactly the same. I have to bend the string up at the 14th fret to get the right pitch. Does anyone know what is causing this and how it can be fixed? Thanks.

it's the same sort of thing on my sisters guitar and it means that when you press down on the 14 fret it doesn't touch the 14th fret but touches the 15, your 14 fret is to low and if you take it to a guitar shop they should be able to fix it.