A potential explanation I've recently heard is that, due to the impossibly fast, silent maneuvers made by UFOs, is that maybe they have Alcubierre drives - they don't move through space, they move space around themselves, enabling moves at any acceleration/speed without the ship experiencing the utterly brutal acceleration observed. This localized bending of spacetime would create gravitational lenses that obscure their true shape, so a "clear" picture is not possible unless one turned off their engine, in which case it would be in freefall, a clumsy move. The sharpest footage yet seems to be from CBP in Puerto Rico in 2013, you can see the footage here: https://www.youtube.com/watch?v=ldV5LUsTkJM

The description on the video has a link to a page that goes into much more detail. Obviously the page gives similar vibes to any (other?) conspiracy theory page, but the explanation seems more consistent than any other I've heard.