got new mac, all song purchased on the former one are no longer in the library - how can I get them, please? thx

They aren't there because you didn't transfer them.

Downloading (using iOS or computer) past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not all media formats are available in all countries (see: iTunes in the Cloud, iTunes Match, and iTunes Radio availability by country - http://support.apple.com/kb/HT5085); apps, books (not audiobooks), music, t.v. shows, and movies (some - not all studios have permitted this). Downloading previously purchased movies and TV shows requires iTunes 10.6 or later. Discontinued items not available. For items not included in the iCloud list (e.g., ringtones, audiobooks), or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple recommends, "You always back up your iTunes library in the event that a purchased item is no longer available on the iTunes Store," (http://support.apple.com/kb/ht2519).

Additional tip at https://discussions.apple.com/thread/5680301?answerId=24176756022#24176756022

In future do not rely upon being able to download again. The policy could be discontinued without notice and then you'd be in a mess. Even now you still can't redownload everything. Transfer properly next time and make backups.

I doubt anybody is gong to be able to tell you why because there is insufficient information on what you have been doing the past 4 months. There's no new policies. I have music that has been on my computer for 20 years so it really is a matter of how you manage things. Yes, there's a couple of posts a week here where people have music that has disappeared without explanation, but I suspect part of the lack of explanation isn't what iTunes is doing but what the user is doing without realizing it. All that said, the only long-term insurance against disappearing music is to make sure you have backups of everything at all times. I write over my old backups but before I do so I make sure that my current library is complete.