Around the rings are there any mushrooms or puffball fungi? If so this could be a fairy ring. Fairy rings are difficult to control:(
~ But if there isn't any signs of mushrooms or puffballs it coul be a large patch. A way to manage this would be to aerify and dethatch regularly to keep thatch at1/2 inch or less. fungicides are most effective in the fall for prevention.

I don't think that you have fairy problems, lol, I think that it is the structor of the soil' it depends on what was there befor the lawrn was layed, I have dents in the lawrn that dose the same thing, it means that that part is geting more water than the rest of the lawrn.

did have trouble with mushrooms last year but do not seem to be the places where this problem has occurred. thanks for responses will check both out.

the thick grass is as a result of the toadstool releasing nutrients from growth last year. the ring will get larger and larger as the fungus spreads. it wont do any harm unless you want a perfect lawn. :o)