Hi! You've been working hard and I admire your patience with this. We're going to get this, I promise!
I'm not a DP-24 owner (at least not yet), so I'm not experienced with the particulars of the settings/functions of the mastering mode of this unit. I've downloaded a copy of the manual and here's my best guess so far:
I think that some of the quickest and smallest peaks are sneaking past your compressor settings, leaving your DP-24 with very little extra room to make your song louder during Normalization.
For a visual, go to this link for an instant (
http://www.auldworks.com/articles/webdyn1.htm). There are two diagrams shown in this article.
Figure 1 - To little compression
Figure 2 - To much compression
Looking at Figure 1, you'll see that there are a couple of peaks that are higher than the rest of the entire song. If just a few of these could be QUICKLY reduced to the level of the rest of the song, Normalization would be able to raise the overall level much more. (P.S. Too much compression/limiting will ruin the sound of your music as possibly shown in Figure 2).
For a beginning setting on your DP-24 mastering compressor, try the following (this is, of course, very subjective):
Single Band compression
Ratio - Max
Attack - minimum
Release - 30 ms
Knee - middle-ish
Threshold - set so that you get gain reduction (GR on the compressors meter) of no more than 3 or 4 dB. You actually won't see it doing a whole lot, just clipping off a few pesky peaks. This type of setting is considered a "brick wall" limiter and won't really affect the sound of your mix a whole lot, just make room for it to be louder.
I'd suggest EQ'ing your mix before Limiting in this fashion,then normalize.
If there is some confusion with the DP-24, you can also do this same type of process with Audacity. If you find yourself at your wits end, I'd be happy to "master" your tracks for you and send them back, if you'd like.
Let me know what you find.