How to prevent design rejection

There should be not white border around your design. Ideally, the artboard is as big as your design, and your design is clipped to the size of your artboard.

You have several design elements that have (nearly) the same colour as the background, that makes that those are not visible.

The asset is grouped, so that is a good thing.

In my view, it would be more profitable for you to submit the single vector graphics  (megaphone, target) than the complete design.

What was the rejection reason?

The devil is in the details.

Examine images at increased magnification. 

Vector graphics should have crisp edges & sharp details at any scale (small or large).

Some poorly drawn elements.

The edges of your designs are very jagged.