I had a short look into your designs and I guess the issue is as follows:
some of your designs have a half transparent shadow-layer. Avoid Halftransparencies and those designs will probably be acepted. Most of the other designs I checked were pixelated. Although all of your designs meet our requirements in terms of solution or pixelsize I guess you just took low quality images and scaled them up. This gives the designs the very unattractive pixelation effect.
The technical data of a file alone say nothing about the visual quality of a design.
You only have to zoom into your designs once, then you can see relatively quickly that the quality is not very good.
Some elements are very blurred, others are pixelated. Here is just a small screenshot of one of your designs.
I hope this helps.