Why can't I listen to some songs on iTunes Store before buying?

Couldn’t listen to the preview on iTunes but once i bought it, i could obviously listen to the whole song.

Indeed you're correct, I just tested it and couldn't listen to it here either. It continues to search for a connection.

According to the system status, there is no problem with Apple.

Apple - Support - System Status

Based on Downdetector -> https://downdetector.com/status/apple-support/

Maybe try again in an hour or so?

Or get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

It's strange, I keep testing different Album/songs, and some Album/songs do play.

For example:

Fleedwood Mac - Greatest Hits & Rolling Stones - Hot Rcks 1964-1971, when I click at Preview All I can listen the whole Album.

The Rolling Stones Singles Collection - The London Years -> Preview All does not play.

A question: do you see that Album in your Music Library?