Hi,
Could be one of several reasons, here's a few :
1 - Make sure that you DON'T use the "volume control" that appears in the bottom
right of the main arrange page, ( small left/right slider) this directly effects the
main stereo master output and will reduce any bounced file's level.
2 - Are use using "bounce in place" ? for individual tracks it's useful BUT will keep
the level of that track, so if it's reduced a lot = low level !
3 - Is your main stereo out showing a fairly healthy level whilst your song is
running and before you bounce ?
Hope that helps ?
M.