If you spent $8000 on sod, what does the company that installed it say?
I certainly would have them looking at it and giving an opinion. Did
you get any kind of guarantee? It's impossible to diagnose this type
of thing remotely. It can even be difficult to diagnose by inspection.
You say you were not advised to add top soil, but how much top soil is
there? You should have at least 6 inchs of decent soil, 8 is even
better. How frequently are you watering it? Too much water and
nitrogen can do harm by promoting fungus and disease, particulary in
hot weather. At this point, you should probably be watering it twice a
week, with about 3/4 inch at a time, unless it rains. It should not
be constantly wet or soggy.

At this point, I'd contact a local agricultural extension, if they have
them in your area. In many areas, these are free and you can typically
find them in the county govt listings. You can take them a sample and
they will give an opinion.

I would also ask them for a recommendation for a turf consultant. You
can also find them listed in the yellow pages or the internet. A good
one may charge $500 for a visit, but if you've got $8000 on the line, I
would get one real fast. These guys are typically professors at agri
colleges, ex-professors, etc. with lots of experience. They usually
consult for golf courses, sports fields, parks, etc. Check their
credentials. If it turns out there was something wrong with the sod or
how it was sold, their expert opinion could be needed if you want to
try to recover money from the sod installer.