Does Apple Music let you hear the whole song on iPhone and iPad? (instead of just 30 seconds or 1:30 of sampling) After I started with the Apple Music 3-month trial, sometimes I search for a song on my iPhone, and I could actually hear the whole song.

So I thought Apple Music probably lets me hear the whole song for most or all of its songs, but there are other cases when I search for a song on my Macbook using iTunes, I could only hear the 30 second clip or 1.5 minute clip like before.

So what is the rule?  I could hear most (or all) the songs on iPhone and iPad for any song on iTunes Store, but not on  any Mac -- is this basically the rule for subscribers to Apple Music?
I've never had a song not play through on iOS or OS X via Apple Music. My guess is network congestion is the culprit when you don't get a full play. 

Another possibility is your iTunes account is in a geographical region where Apple Music doesnt have broad coverage of the songs in Apple's total catalog. In at case, those songs that are for sale somewhere in the world, but not in your region, might have previews instead of full plays. 

For me in the us, Apple Music gives me all the music I care to consume for the subscription period across all decides that are signed your Apple ID. The notorious artists like Prince simply don't show up so I haven't found a song that' soft sale but not for streaming.