I have noticed over the months that impact does not work on certain enemies. Has anyone else noticed this as well?

Case in point: My Level 27 Mage went to Guldun Rock near Valthiem Towers in Whiterun Hold. There were two giants there, as usual. I shot one with dual cast Ice Spike, then the other. I noticed that one got staggered immediately, but the other one kept charging at me at high speed. Dual Impact stagger didn't work for him. I tried it 6 more times with him, but still no stagger, he slowed down eventually with the ice slow effect. I have noticed this in other situations too. Certain enemies do not get staggered. Some dragons get staggered with dual cast. There are some that do not get staggered.

Isn't Impact supposed to work all the time?

I haven't noticed much help from impact at all. I think destruction magic in general is pretty crippled. I just can't have fun playing a mage character. Yeah, I can cast illusion magic all day long and sneak around and conjure things to do my work for me and travel with a whole entourage of followers, but it just doesn't seem as much fun as a melee character that way.

I haven't experienced what you are describing but I have seen impact not work. I see it a lot with bears. If you hit them as they are charging sometimes they just run through it. For bears I've noticed it depends on when you hit them as they are running. It seems one of the 'positions' they are in as they are running can't be stunned, but if you hit them half a second earlier or later they stun. I've fought a lot of bears lately and I can actually tell right before the spell hits when it isn't going to stagger them.

I have seen the same thing with giants. If you hit them at a certain time when they are charging they run through it.

I think the issue is that the particular movement they are doing doesn't have scripting for 'stopping'. Dragons for instance can't be staggered when they are flying forward, they just flap their wings funny as they keep going. However if you hit a dragon while it is hovering in the air or on the ground it can be staggered.

I think you might have just had a weird sequence where you were constantly hitting the giant in his 'non-stagger' pose over and over.

This makes sense to me. Perhaps you are right. I will look at this more carefully. Thank you for your input.

I have also noticed that after an enemy is staggered, there is a moment when they are recovering they do not take any damage. I also noticed that Bears recover from stagger much much faster than a giant. Have you perhaps noticed this as well?

You are not the only one who thinks that. There are at least 6 guys on this forum who post everyday about how terrible Destruction is. Don't they have anything better to do?

I disagree with them. I think magic is effective and fun. I think destruction is effective and fun. Pure mage is my favorite style of play at the moment. Destruction is particularly strong in early game and I tend to build mage characters centered on destruction.

You just need to learn a few things.