Too much or not enough of (water, sun). Or not enough good soil base, or wrong lawn type. Planning is essential although some mistakes can be corrected later. After that? Good care: mow high, leave debris (mulch), nourish and water intelligently. Personally, using a good watering system (more than programmable: « intelligent » timer and sprinkler system) helped me a lot! I coupled it with a weather system and not only saved tons of water but enjoy a beautiful and soft lawn!