There are quite a few causes of bare patches in grass - dog pee, 'scalping' higher spots with the mower (which may themselves be caused by ants' nests), or dollar spot disease, fusarium patch disease, red thread, applying mosskiller treatment which kills the moss and leaves bald patches, uneven application of lawn fertilizer killing of some areas, so more info required for a diagnosis.

thank you Bamboo, i have made a note of all the things suggested and will try and find which one is the culprit.