You could be forever finding out what's causing this.
For what it's worth, I'd start again and re-turf it, if you aren't bothered about the cost .
I'd remove all the old grass. Rotovate the soil, level it off and compact it down a bit.. I did this to our garden thirty years ago. (the back half, although you wouldn't know it, was originally a vegetable patch).
Then buy some new turf.
I'd suggest (if you want to do this) you be a bit choosy about from where you purchase it, as at garden centres, it's sometimes poor quality but cheap. Occasionally over the years, I've had to replace some under acers and azaleas, as the shade encourage moss to grow.
So whenever I've needed two or three rolls, I find out what day it's being delivered and buy it on that day, as often it sits on pallets outside garden centres in the sun for a week.
I'm a firm believer of frequent cutting, I even cut it during the winter. I set my Flymo on the middle height. I use cheap Wilko's lawn food and iron sulphate occasionally, on moss in a few places along the long border. It's never much bother.