Is "bouncing" something that needs to be done, anyway?

I did a mixdown from the midi notes - the whole song, and some particular notes are not heard in the mixdown / wav file. Went back into song / track / instrument, thinking I deleted the note by accident, but it was there. -Played the section again, right there, and nope - no notes played. At any BPM.

I have not, however, done this bouncing thing. Still low in the leaing curve. I'll check that out and if no go, I'll back off a version and wait for the update. Thanks for the info ^^, Presonus people!