First of all, Thank you all.
With Henry's hint, I found that it is not really connected for the upper polyline.
Andy, I can't recall how the double nodes occurred, as I make it the same way as lower profile.
I could only think of when I trimmed the unwanted parts, I made some mistakes there.
My finding, with Henry's hint
, is the same as Don's.
After exploded once, I use 'chain polyline', as attached, and found the disconnection.
After use 'meet 2 lines', then rejoin, it works.
I am not sure if it is a right way or any other better approaches to find the disconnection, but it works for me.
Any suggestion on finding disconnection (more efficient way) would be appreciated.
Thanks again, all.
Simon , a happy user