First of all, thanks for your reply.
Unfortunately, the roofing contractors couldn't come today.. they'll
come to my place tomorrow.. but we are pretty sure that the issue is
the flashing or the vinyl is ripped.

I actually understand that it was hard for the inspector to find that
problem. Indeed, the swelled moulding part was covered by some
furniture at that time.
Who i'm mad at is the previous home-owner. The previous owner didn't
live in this house, he just rent out to some other family. But
shouldn't the previous owner have checked any damage and report to us
after the tenent moved out?
I assume the owner should have noticed it because the middle part of
the swelled moulding turned to yellow from white colour.
Bad for us, the previous owner lives in another province, making very
hard for us to get a hold on him. My family kinda decided to pay for
the repair and give up to find who's fault it is. It seems like more we
try to find it out, the more we ruin other ppl's feeling as well as
ours.

Sometimes, in my experience, things happen -- although typically, the
roof only starts leaking after you've placed something really really
valuable/fragile under the point where it starts leaking....

I totally agree with u Caledonia.. things just happen.. we just got a
bad luck this time..