Headphone Mic not Working on iPhone 6

Hello.

I've an iPhone 6. When I connect any one of my headphones, I can listen to music or hear the other side during a phone call but the mic does not function at all. The other side can't hear me or I can't command Siri.

I tried force rebooting my iPhone. Sometimes it helps but the above situation keeps occurring.

I've opened my iPhone myself several times for battery and screen replacements. After all of these repairs, it was fully functional; including the headphone mic. This problem started recently.

What can I do to diagnose and repair the problem? I'm thinking about a full reset but I've concerns about the hardware. Could this be a hardware problem caused accidentally by one of my repairs that occurred now or maybe a fall caused it?

Thanks.