Does it happen if you render an individual track to wave, or just the when you render all tracks to wave?
What is the synth you are using to render the song through?

I know I had to futz around a bit to get my Focusrite devices to work as expected in RB/PT, but that was mostly making sure I had matching bitrates and a good USB cable.
The Focusrite supplied USB cabled worked as expected but when I tried a longer one I had on hand it choked in unexpected ways..
Once I bought a good cable things started behaving as expected.
I think I splurged a whole $15 on the new cable. <grin>
It was a bugger to solve at first, almost returned the unit(s)


I do not work here, but the benefits are still awesome
Make your sound your own!