Explanation
Because it is chosen in a full-screen editor and viewed at roughly the size of a postage stamp, most cover frames are unreadable in the only places they matter. At grid size only large shapes and strong contrast survive, so a beautifully composed frame in subtle tones reads as a grey square.
Profile grids crop to square on all three platforms while feeds do not, which means a cover composed for vertical can lose its text entirely on the profile. That single crop difference accounts for a large share of covers that look fine in the editor and wrong in place.
The common mistake
Judging a cover at full size. Shrink it to grid dimensions before choosing, and cut the text to three or four words, because anything longer becomes texture.