Weird png issue

I had been using pngs on my dts models (exported from Maya) with no poblems. However recently I occasionaly will not be able to get my png textures to show up on my dts geometry within torque. In some instances I had the same problem with the textures from the fps demo png being visible on my geometry. One solution I found to fix the problem was, in photoshop, to add an alpha channel without the transparency. What I do not understand is why some png textres will display without the alpha channel while others will not unless I add it. Is this a bug or am I doing soomething wrong?