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Most units such as yours are controlled by a defrost timer. This is probably the cause of your unit not defrosting. Of course there are other things that can cause a no defrost condition to occure also.
What depressing the light switch has to do with the defrost cycle I have no clue of. Is this something that a tech told you to do? Or did you read it somewhere?
Can you tell me if you hear a fan come on if you allow the temp to increase inside the unit? It will usually coincide with the compressor turning on.
At present is there a lot of frost buildup in the freezer section?
Any ice or water puddling at the bottom of the freezer or refer section?
If you would please answer these few questions I will be able to provide you with the best course of action to resolve your problem.
Best regarsds; THS
Please do not accept this as it is not a complete answer to your question, and I possibly may have to opt out if I do not have a solution to your problem.