Pinupkate - I know what you mean - I had that problem with some of mine.
I think it's a combination of things that causes it - too much glue; not letting it dry properly before adding magnet; quality/absorbency of paper; and the clearness of the glass (but you can usually see that before glueing).
I've made hundreds and hundreds of these, and I've found that I just need to allow for up to 10 per cent to not be perfect.
If they go cloudy like that, I just soak off the picture and reuse the glass, so not much is really wasted.
It's really just trial and error..... and practice, practice, practice!
But as it's happened to so many of yours, I think you're probably not letting them dry between layers (and maybe applying too much glue).