James,
I can't tell exactly how you have that setup unless you share definition, or you share to charthub. However, a few things that could be going on there...you might have the Delta bars drawing behind the Buy and Sell Volume bars...and therefore getting covered up. You can either right-click in the pane title, or shift-click in the horizontal scale to the right, to bring up the element order prefs window. Try adjusting the element order. You can also right-click on any histogram and choose "send to back".
Other problem could be that you just have the colors set the same for both the buy volume and the positive delta. Try adjusting the colors of the different VBs so they stand out.
If none of this works, share your chart def and I'll take a look.
Here are instructions on sharing a chart if you're not familiar with that:
http://www.linnsoft.com/qa/a/124.htm