Too much detail
Complex illustrations can look polished on a large canvas but collapse at launcher size. Reduce the artwork to a recognisable silhouette and a few high-contrast shapes.
Weak contrast and unsafe edges
A low-contrast subject disappears against wallpapers and Play surfaces. Test both light and dark surroundings, and keep important elements away from outer edges so masks and crops do not remove them.
Starting from a small export
Do not enlarge a small PNG to create a new deliverable. Keep a high-resolution source, make changes there, and generate all sizes from that master.
Google Play requirements can change. Verify current details in the official Play Console preview-assets guidance and in your Play Console before publishing.