Need Help with Logic Express

[Hazy Davey]

Hello;
Yestrday I've upgraded to Mac OS X 10.7.3
Needed to upgrade Logic Express after that, downloaded and installed upgrade, now using Logic Express 9.1.7
Unfortunately ran into a problem which never had before:
When I'm adding new track, and try to jam with prevously recorded tracks, there is sound only either on my newly created track or the rest of tracks. Can't hear both tracks simultaneously. Need to point out that even when there is no sound, level indicator is showing that there is a sygnal on the track.
Sorry it may sound little confusing, but those who use Logic would understand.
Thanks