Common failure modes include bass notes that imply the wrong root, vocals that mask chord color, distorted guitars, live timing drift, short passing chords, and recordings that differ from the tab everyone shares.
Dense mixes
Many instruments can blur the chord tones the app needs.
Timing ambiguity
The chord may be right but the boundary may land early or late.
Musical context
Borrowed chords and inversions need more than a local pitch snapshot.