I have had that problem with a midi file: I opened a midi file in RealBand and added RealTracks for accompaniment. It was always out of sync.
So I got the sheet music and entered all the notes in BIAB, gave it a style for testing. Everything is o.k. And then I open the file in RB. Melody and Band are out of sync.
Here is one file for testing:
https://dl.dropboxusercontent.com/u/37640589/Thorsa%20V6.MGUEdit: It starts in sync, stays in sync in BIAB and gets out of sync at measure 34 in RB
Background: My brother and I discovered an old scratch book with a couple of tunes our dad composed in the 1950s. They are not very good, but they are his. We wanted to give him a CD for Christmas.
I do have the latest recommended build of all the programs