As far as I can tell, the little glass skylight filter I have on my standard lens doesnt make any difference. Also, the focus of the standard lens doenst matter either, except in how close you need to get to the subject to get a clear focus. Perhaps you arent getting close enough? Im finding that the Diana lens is about the same as the Holga ML-60 close up lens and when using either, you have to get like an inch away from your subject. The 120 gives you more distance from your subject and is why I use it most.
And on another note, Im going to make another home made lens with the Holga ML-60 and see how that works out for me.