I'm not sure what the factors influencing the effectiveness of dispel are. I have noticed that in late levels, with all kinds of upgrades that interact in different ways, dispels are a whole lot easier late in the game than they are in the beginning. In the Emerald Graves, I seldom even see the second wave. There's some kind of level shift in the Emprise, and dispels go back to being average. I have noticed that certain combinations of equipment make changes in attributes that I didn't expect, and these have an impact in ways I didn't realize. For me, it was the reaver tree with Bull. He was constantly dying until I did something or another different, and now he's nearly unstoppable. I might have built him different. I've been playing with a more right-brain approach, looking for the patterns rather than looking for the facts, because that seems to work best for me, so I can't say exactly what it was I did differently.
The advice I'd offer is to check his attributes and see if anything looks bizarre. I wondered why Cass kept dying until I unequipped Wintersbreath, after seeing that with it, she was starting out at -90% resistance to everything. She did better at staying alive than she did at killing others, and my attempts to undo it, the way she was built, intensified the problem. When I gave her a shield that did what she needed, the way I had built her, she rejoined the party as the righteous defender I had in mind. It might be as easy as a piece of gear or playstyle that's giving you grief, and it's probably a safe idea to check there first.