is it a new build or a SH house? Ive same problem with my back lawn but its because the previous owner had built an extension and for some reason used a layer of rubble to put the lawn down on, pick a small area and dig down as far as you can go to see if there is a hard layer that might be recreating the same effect.
Its so bad on my lawn that if it rains on a monday its still damp on the friday and thats even with it slanting away down to 30ft trees, so seriously thinking of digging up and relaying. If we all came round on a saturday Im sure we'd be finished in no time.....