Power button half working

The power button suddenly became very strange, after not using my MBA for a while (actually used by my mum).

It's very difficult to power it on when it's shut down, need to try pressing it many many time and suddenly it may power on the MBA.

However, if I did not shutdown the MBA but put it to sleep instead, the power button can always turn it back on.

So it doesn't look like a hardware problem with the power button right? I've tried SMC reset but it doesn't help either.

Any idea?

THX