Strange behavior of laptop keyboard key [Hardware troubleshoot]

Hi

I have a curious case with my laptop keyboard. When the keyboard is idle for some minutes (eg, 20, 30 minutes or more), at first re-use, the "n" key will not work, unless I press it really really really hard... However, the more I use the "n" key, the less I need to press it hardly... After some minutes of using the "n" key, it will become completely functional like all other keys (activated with the slightest pressure)...

Once I put the laptop aside for say half an hour and then want to use it again, the same problem happens.

Why does it happen? Why does it get fixed after some time of usage?!

And how can I fix it?

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Additional information (please read the followings before asking me questions about my laptop):

1. I have put an extra plastic cover on my keyboard (from the beginning of its usage) that prevents spilling any liquids or other things on the keys. The interesting point is that my strokes on the "n" and "h" keys are so strong (and with my nail rather than my finger pad) that the sheet over these two keys is torn.

2. No one else has access to my laptop, and I am sure if there is any physical damage, it is not by some liquids spilled over the keyboard. I can also assure you there is no small particles underneath the "n" key. Perhaps the strokes of my nail might be the culprit.

3. The problem seems not software oriented, as I see it within different programs and after resetting the computer. Also the OnScreen Keyboard "n" works properly, as expected.

4. Besides, the Fn key does not have anything to do with the "n" key.

5. This happens only with the "n" key, and not even the "h" key (the other key with a torn plastic sheet) or any other keys.

My laptop is Acer Extensa 5635G, and I am using it somehow non-stop for the last 4 years.