When the needles dropped by conifer trees break down naturally, they release compounds which turn the soil acidic. To an extent that they kill other plants and the forest floor in a conifer plantation looks dead, compared to a deciduous wood's undergrowth.
Conifers prefer acid soil, so they effectively prepare the ground for their progeny.
Grass is notorious for out-competing slower growing plants so, if the grass is struggling, I take that as a sign that the soil condition needs assessing and remedial work will be needed before anything else goes in that patch.