I wonder if by "tapping the carp out of it", you broke it...
Do some systematic testing!
Plug the mic into something else (ideally a location mixer) and gently test it. Stand it close to a radio (for a reasonably constant sound source), and listen to it on decent headphones, to check it's not intermittent (a meter alone may not tell you), and that the output level is what you expect. Wiggle the cable and gently pull the cable behind the connector(s) to check for breaks (often they reconnect, when the two broken ends touch). Do all this with a fresh battery in the mic.
Check you've done nothing on the camera to switch off or turn down its audio inputs. In the case of ML, boot without ML and try the audio inputs, then with it.
If you haven't got one, buy or borrow a tone source (audio oscillator), and if it hasn't got mic-level output, buy or make a 60dB pad (attenuator), to drop its output down to mic level from line. Feed that into the camera, and check the camera's audio inputs. Google is your friend - the components literally cost pennies.
Domestic "stereo" audio leads, with a 3.5mm plug broken out to two RCA plugs ("phono") are helpful for this sort of thing, as it's fairly easy to get a cable from an oscillator to the phonos, and phonos are very reliable. Those leads often have better-made 3.5mm plugs than the solderable sort.
1kHz tone at -60dB* should be middle of the range level for a decent mic input (my 6D has up to 70dB gain, and fairly quiet mic amps which is very good for semi-domestic kit). It should give you a stable reading on the meter. GENTLY wiggle the plug in the camer, in case the socket is failing - look for the meters to drop out, on one channel or both. If that is happening, your camera may need electrico-mechanical repair (the socket will need replacing). There's no easy fix I know of: 3.5mm plugs and sockets are horrid things, used for cheapness really. There's a good reason why Cannon XLR plugs have been popular for 60+ years!
If your camera provides low voltage DC power for mics, and your mic needs it, take your 3.5mm plug-to-phono breakout lead and test on the phonos with a digital multimeter that there are volts available (I think between tip and sleeve, but personally I never use mics that way, as it's a nasty powering system - you will probably only get around 3-5V on one phono alone (can't remember the wiring, to be honest)).
The tip and ring of the 3.5mm plug go to the pins of the two phono plugs, and both the shields (screens) go to the sleeve.
Try all that, and please report back what you found.
PS: You probably realise I'm on the side of your mic in this generally: imagine someone tapping hard on one of your ears! To test a mic all you need to do is rub your fingernail on the windshield gently. If that doesn't make the meter move, you have a problem somewhere.
To test recorders, tone sources are a much better idea than mics, as tone is consistent, and you can concentrate on the recorder, and not worry about the mic. You can also hear distortion, etc, far more easily. Once you have an oscillator you'll wonder how you ever managed without one when setting up audio stuff.
*There is a 10dB difference between "line" level on domestic hifi and proper line level: domestic kit is more sensitive. Microphone specifications usually use the professional zero level value, so -60dB from a mic will be -50dB on amateur kit, such as most of Canon's cameras and on non-professional recorders. An if an oscillator (tone source) has several different output connectors (e.g. XLR and phonos or XLR and a 3.5mm socket), it may take this into acount or it may not -- if all else fails read the instructions or use a meter to find out!