there's up to 3 solo / mutes modes in Logic:
Track mute mode (the m is highlighted on the track header in the arrange) and mutes all regions on the track.
Region Solo mode. Press S on the keyboard or click on the solo button on the transport. Any regions selected will be soloed and have a yellow border to show they are in solo mode. If a region is not soled you will not hear anything
Mixer Mute mode. Open the Mixer and check that a track is not muted in this mode. Mixer solo/mute is an instant tool.
Also, check that your midi part that is not playing back has the correct output 1-2 selected in the I/O section and the instrument is not bypassed.
Let us know if this helps,