Dhav. I get this problem with xfrog leaf image maps all the time. I don't even bother using them anymore, I just convert them to JPGs and create a seperate transparency map (also JPG). Some of the xfrog tiffs won't even open in certain image editors. I think
paint.net will take them ok. I don't think its a problem with the alpha chanel in particular, (I will do a close up render, and the shape of the leaf is there its just has white facets on its surface) I think its an incorrect tag value (or more than one) like Mandrake said. Try opening them in an image editor and resave.
Ta
Miles