Grass - Greeness Issue

How long has the lawn been down, did you sow it yourself or did you inherit it? It could be something under the lawn which is restricting growth, identifying exactly what depends on the history of the site. Any chance of a pic?

Could the yellow patches relate to the major roots of the trees that are near the surface? I am not sure why this would suddenly start making the grass turn yellow unless there had been little or no grass there before because of shade, and now it is noticeable.

Were the stumps poisoned I wonder?

But don't spread it now. It's too warm and too dry. Wait until autumn or spring.