Question
Tom,
My recently cleared back yard (probably 20 x 80) strip allows for my Pennington Grass seeds to flourish into beautiful, lush, green grass.  After a while, though, the grass literally disappears.  After I cut it or not, it disappears.  I don't know if my lawnmower blade needs to be replaced (probably does), but whole patches-worth of grass just systematically disappear, and I end up re-seading that area (it grows, but then dies again).  Any thoughts?  Yes, we are in a drought-style season down here in Atlanta.  Yes, I don't know what the hell I'm doing (first year I ever clear land, didn't even test the soil, it's not Georgia Red Clay though, seed, and the stuff grows).  That spot gets enough sun, enough shade, enough humidity.  I was watering once a day, and it worked great, and then the patches showed up.  Then I stopped watering some areas, and the grass stopped growing again.  Sorry for the long post.  Thanks again for your time.  
Oscar.

Answer
Oscar:

Thank you for the detailed description of the problem and history.  In most questions, I am not told where, when, and how.

It may be that there is a fungus or some problem with the soil.  From your description of the repeated failure, I suspect that a fungus would act like that.  You can reasonably treat with a fungicide and sew again.  If it continues to grow, then, the problem is solved.  Patches are signs of fungus.

You may also take soil and plant tissue samples to the local ag agent to see what tests reveal.  A tissue sample should include roots and tops of several plants.

If no soil was added, then I would guess that no phytotoxin was introduced.  A soil test with that parameter specified would reveal a toxin to plants, like a heavy metal.

In this case, the soil would have to be removed and replaced.  This will not get rid of a fungus.

Pennington is a good brand of seed and is certified.  There should be no problem with it.

Best wishes.