Seems like a lot of fuss about the energy shield on Hoth. The way I see it it works like the one used during the Battle of Naboo : it is designed to prevent blaster bolt from penetrating it but can't stop something moving slowly to penetrate it. What ady could maybe add is some kind of effect (some sort of hue or glow, whatever works) on the surface of the AT-ATs right before they start firing, hinting the fact that they've just penetrated the shield and thus are now able to fire.

Also a very good point as been made earlier about how easily Luke escapes Hoth despite the imperial fleet, here's how maybe it can be worked around : we know that at least one cruiser as been disabled by the rebel Ion cannon, Ady could easily enough add a disabled star destroyer in the shot when Luke flies off the planet.