Your issue sounds a little different than mine where the mic never works. Looking at dmesg is a good start. Reproduce the problem as you said you could earlier. Look for any of the "switches" that activate a different audio profile. (IE: Headphone now connected etc.)

When the mic isn't working launch alasamixer ( alsamixer -c0 for full view) Check and record all the settings to compare against when the mic is working.

When the mic is not working try changing audio profiles either by inserting a headset or changing the profile to silent etc. See if that corrects the issue.

Unless you've got an application overriding the mixer settings or something strange like that I'd start looking into my repair / replacement or return options.

Brian