I can't hear everything in FL Studio?

My sound is on, I know the volume is right in FL because when I click a note on the piano roll, I can hear it, but when i click PLAY on the playback, it doesn't play what I selected from the piano roll. It shows the green lines of the piano notes in the pattern row where the piano instrument is. I can hear the beats that I made, but not the notes for the piano? Why is this?