The warm water for your washer is simply a mixture of the hot and cold water available from your home. If the hot water entering the machine isn't very hot, the warm is actually cool. Also, in northern climates, during winter months, the cold water entering the unit may be significantly colder than in the summer, which causes the warm water to be cooler.
In other cases, the water inlet valve may be restricted, or there may be sediment on the screen, that blocks the input of the hot water. Check out the washing machine troubleshooting and repair help information for additional information.