Air conditioner blows hot air. Why?

It most likely is leaking, The leak needs to be found and fixed. Do not use the DIY Air Conditioner Leak-stop garbage as these will eventually clog up other things in the system resulting in more repairs.

Check this information of A/C that blows hot air http://www.cartechhome.com/2008/07/your-car-air-conditioning-system-is.html

the refrigerant is oviously leaking out, check your high side hose, they commonly leak, if not visible you may have to add dye to the system next time you fill it up, then when it leaks out again use a uv light to find the leaks