Hi, what is happening is the defrost control board or defrost thermostat has a problem. It works on time and temperature to defrost and its not doing it by or I should say on its own at the times needed. This is why you have to do it manually, to reverse the flow of freon to melt the Ice. You would have to be a qualified service tech to trouble shoot the unit to see if the problem is in the defrost board for timed defrost, or the snap on defrost thermostat for a temperature defrost. If you call a tech out, it will cost you some $$$. What I would do if you want to take care of this, it to call around to a/c places and appliance repair houses and ask what they would charge you if you removed it with some help from a friend or two, and take it in to them. If you do go this route, make sure you ask if its going to set on a bench somewhere for a week before they even look at it also. Make sure they give you a written estimate on parts and labor and compare that to a new unit. I wish I could walk you through this, but there are so many controls to check and you would need a set of manifold gauges to even attemp a repair and to be able to read these to know where to go. I hope that I have given you some direction to go to, and have been of help to you. Please be kind in rating if you will, and I will always be here for any help you may need one way or another. Thanks for using us here at FixYa.com
Sincerely,
Shastalaker7