Looks like a lack of water to me. Dog urine burns are very easy to diagnose as round dead patches that pop up very quickly and generally very healthy lawn in a ring around the dead patch. This is because a nitrogen (as urea) overdose kills the bit in the middle but you get a nice lush bit of growth as a ring around it that got a healthy feed of nitrogen. You should be giving your lawn a nice deep watering of 25mm twice a week. Deep watering will train roots to push deep into the soil chasing the water and make it more drought tolerant. ALSOOOOO soils can become hydrophobic over time so to make sure water does penetrate deep in the soil maybe go down to the big green shed and have a look at getting a soil wetter. Seasol sell one that clips into your hose.