Power button working intermittently (have to press hard)

Hi all, my 21 month old OnePlus one has become increasingly difficult to switch on using the power button. It now seems to require quite a bit of pressure, and still only then works intermittently. It certainly appears to be a hardware issue. Any ideas what could be wrong, and the best approach for fixing it. Other then this, the phone is perfect, and has plenty of life in it yet. Thanks.