It's still pretty early in the growing season (in the Northern hemisphere), so the thatch may not have had sufficient time to break down and give way to the new growth. 

But from the look of your pictures (great job, by the way), if you feel the lawn is struggling to fill in those dead spots, your lawn looks like it needs a good de-thatching more than aeration. 

De-thatching is the process of removing some of that dead material to make way for new growth, where aeration is recommended more for when dead material or compacted soil is keeping water/nutrients from penetrating the soil. It's hard to say from the photos, but I'm not seeing an aeration problem here. 

Another thing to watch for is the possibility of fungus. I'm dealing with a   southern lawn, but the pattern looks remarkably similar. Keep an eye on it; if it looks like the brown areas are actually starting to *spread*, it might be time to diagnose exactly what you are dealing with in terms of some type of fungus or other lawn pest or disease. 

Start with a rake or dedicated de-thatch tool to remove some of that dead material.  No matter what the problem turns out to be, it's a good time of year to start that process.