It would seem you have to fire decals on a low heat, in the microwave kiln, so you need a bit of extra patience - (which I haven't really got :o) ) .... so I haven't done too
many of them. However, I do like them a lot, so may just have to persevere. Also for a better result - fire them on already fused glass.
The first tomato decal I fired, I fired it on High, and although I rather like the result, it does come out slightly weird!