Thanks to the guys at the SA forums, I was able to figure out my issue. Here's what they told me:
You probably have depth write enabled. If that's the case, it'll appear translucent if it's drawn after the geometry below it, but opaque if it's drawn before because it'll block anything behind it from being drawn.
... for which the solution is to draw all your transparent things last, and, if there's more than one transparent thing that might overlap, draw them in order from far to near.
After putting that fix in it all looks good now :]