Ok, open up the attribute editor with your material selected. On the right hand side of the transparency slider, you'll either see an arrow (connected) or a checker board (not connected). If it shows the arrow, it means that something is controlling the transparency of your material. (It's usually done when you map a texture that has an alpha (transparency) channel associated with it)

To remove this connection (if it exists), right click on the word "Transparency" and click on break connections. Then render it again, and see if that fixes it.

If not, send me the file, and I'll take a look for you.