My rogue's sneak attacks don't work in most scenarios

They work against flatfooted and flanked enemies, but whenever I try to sneak attack against "enemies whose Dex bonus to AC are denied", I get a great bunch of nothing. This trend started happening recently, since entering the ToEE more or less, I don't understand if it's a bug or me missing something about how combat works.

Look at this. Blind AND prone, and yet whenever my rogue attacks him I get no sneak attacks whatsoever. What gives?

 

In the example shown, I believe that the invisibility that the Ogre Chief has prevents sneak attack damage due to invisibility providing concealment. I assume some sort of fog-like spell was cast.

 

Yes, it was Sleet Storm. Is that so? That's the first thing I thought, but I swear my rogue didn't sneak attacks against opponents that were just Prone too. Maybe it was an undead enemy or something?
Speaking of Sleet Storm, I have some difficulty udnerstanding concealment. What's the point of the spell if it makes enemy invisible? s it just an in-built nerf to make the spell balanced, or are there player benefits in having invisible enemies I'm not seeing?

 

Most of those spells slow down the enemy. It is particularly effective if you have AoE spells to launch against them. Also, it can give you extra time to buff or heal your party for when the enemy finally reaches you.

 

Barbarian2 gets Uncanny Dodge.