So, monitoring on an Audio track -- ok, this is most likely to do with the Audio Input Monitoring option (in the Record menu).
You said "when I push play, I can hear the other instruments...but not the keyboard anymore..." If you mean that you're playing along with the (already-recorded tracks in the) song before you start recording, and you have Auto Input Monitoring on (as usual), then that's normal -- when you PLAY without recording, a record-armed track will not monitor incoming audio; when you actually begin recording, however, you should hear the incoming audio as you record. (This is traditional behavior, to facilitate punching on the fly)
If that's it, you can enable input monitoring by clicking on the "I" button at the bottom of the channel strip (right next to the Rec button) -- start playback after enabling "I". This will insure that the incoming audio is passed through when playing/not recording.