Improper irrigation can cause plants to suffer from either too much, or the lack of water. We all know what drought can do to plants. However, many plants will die just as quickly when they are forced to grow in the swampy conditions over-irrigation creates. If your not sure how much water your plants are getting try measuring irrigation output with rain gauges, or straight sided cans. Move the cans or gauges around the yard to check the uniformity of your irrigation.
Most irrigation systems are easy to adjust.
You also may want to inspect irrigation heads. Over time irrigation heads tend to get clogged, and misaligned. Simply cleaning and adjusting these heads can greatly improve irrigation performance. When watering try to encourage deep rooting. Deep, infrequent irrigation encourages plant roots to follow the water down into the soil.