Sounds like your alpha blended quad has zwrite on (this may or may not be what you want). So if you draw the blended quad before the other objects, it blended with the background, and filled in the zbuffer.

But either way, when you alpha blend, you almost always need some kind of sorting.

There are many ways to do things depeding on what you need. You don't necessarily need to sort every object and especially not if you only rendering a few alpha blended quads that don't overlap.

A simple way to start out is by seperating opaque (non-translucent/transparent) objects from non-opaque. Draw all opaque objects first in any order (since they are opaque you don't have to sort, although modern cards prefer rough front to back sorting if you want to squeeze out all the juice. I wouldn't worry about it till later though).

Then test to see if you can just draw your non-opaque items in any order after opaque. for particles, this may be just fine, or if your quads don't overlap much.

If that fails, see if you always have a set order regardless of camera orient, if so, use that. This can also be generalized by giving groups of transparent objects (or just the objects themselves) a fixed sort number and sorting the groups based on that id.

If that fails, you can try and sort objects via distance to the viewer. You can use bounding box centers, closest face/point, or whatever metric fits for you. Use the easiest that works for you. Note, sometimes people don't do exact sorting by distance for every object, but they place them in buckets (with everything in the same bucket being roughly the same general distance from the camera). This can save some sorting time if you have a large number of objects.

If you haven't noticed the basic point is to try and do as little "work" as possible. This isn't just being lazy, it also tends to be the fastest; and it also means you don't waste time doing work you don't need to. You can also combine all the steps to come up with a system that does as little work as it can unless it absolutely has to do more.

Hope that helps some.

I think the z-buffer is what is causing those other objects not to appear, you can turn z-write off but you might get strange results if you don't render your blended objects last.

Somebody [edit: like the post above, doh] that knows more about this than me can give you the details on the best way to sort, but I think the idea is to render your blended objects last, sorted back to front...