No input at all
Check the device is selected and the system has given the app permission. If the input trace is flat, nothing is reaching the engine and no amount of tuning settings will change that.
It says Too loud or Too quiet
The app does not ride the gain for you; it tells you when the level is wrong. Fix it in your system mixer. Too loud means the signal is clipping and the reading will suffer; too quiet means the reed is not clearly above the room.
The note is not recognised, and nothing is playing
You may have played through the startup. The tuner spends about five seconds profiling the room's background noise, and anything sounding during that window is learned as background and then ignored. Stop, wait ten seconds, and play again.
The same thing happens to a note held for about half a minute: it stops being a foreground sound. Stop for a few seconds and carry on.
The reading will not settle
Usually the room, not the app. A reed sounding into a hard room feeds the microphone its own reflections. Get closer, or quieten the room.
Check also that your microphone's driver is not "helping": echo cancellation, noise suppression and stereo enhancement all deform the signal, and a deformed signal cannot be measured. Turn them off in the operating system. Boost, gain and volume are harmless.
The reading wanders by about a hertz
Mains hum. Turn on the 50 or 60 Hz filter.
Two reeds, one number
The pair has merged: they are too close to split into separate peaks. The app shows the beat instead of inventing two numbers. This is not a fault - it is the honest reading of that note, and it is what to tune by.
It happens at the bottom of a musette because a tremolo held constant in cents shrinks in hertz as you go down: 15 cents is 2.3 Hz at C4 but only 1.1 Hz an octave lower. Below about 0.6 Hz a beat is indistinguishable from bellows wobble, and there the app stops rather than guessing.
A musette reads as out when it sounds right
There is probably no beating curve, so the app is measuring against the bare scale. Draw or fit a curve and measure again.
One reed reads out but the tremolo sounds perfect
It may well be. If both reeds of the pair are off by the same amount in the same direction, the beat between them is untouched and nobody can hear it. Judge the beat, not the reed.
The pitch drops when I lean on the bellows
That is the reed, and it is normal - harder pressure swings the tongue further and the pitch falls, worst down low. Tune at the pressure the instrument is played at. Note that the beat barely moves, because both reeds fall together.
The numbers changed when I opened the box
Also normal, and it is the whole reason for the record-then-file workflow. A reed's pitch depends on the cavity it sits in; opening the accordion and lifting the block moves it by around three cents, and putting it back moves it back. Do not file to zero on the bench - see tuning a whole instrument.
Numbers disagree with another tuner
Check the A4 reference on both, and the clock correction. Two tuners with different references are measuring different questions.
The card numbers its columns instead of naming them
The app is not certain which rank each reading came from - the take named no register, or the instrument has no described ranks, or the reading has more figures than that register sounds. It numbers rather than guesses, because a reading filed under the wrong rank looks exactly like one filed under the right one. Describe the instrument's registers and it will name them.