New build lawn was looking good but now starting to deteriorate...

I bought my House [new build] in 1978. The "garden" was just a mass of compacted soil, huge buried lengths of plastic sheeting, timber, lumps of concrete, hundreds of bricks [still digging them up], gravel, lengths of timber, etc. It took me a couple of years plus two lots of 90 plus tons of top soil just to get it any where to look reasonable.
My point being that if your house is new build then you probably have some of the same builders debris buried under a minimal amount of top soil and then it being turfed on top. But grass is a tough survivor and will grow in most places if there is nothing "toxic" in the soil. Having said that it is a bit late in the year to give your grass a top heavy Nitrogen feed but you might try an Autumn feed that will have more Phosphate and Potash in it than Nitrogen. If there is anything under the turf that the grass is reacting to then hopefully time and rain will leach most of it out of the soil.

 

Well, one of the main reasons for aerating the lawn is to alleviate compaction and if you have, as I suspect, builders debris under the turf I would leave it until you get the grass looking stronger and healthier.