Why does vowel harmony in Igbo feel so inconsistent when I’m listening to native speakers?

I’ve been studying the vowel harmony rules (ISO/IHE) for months, but when I listen to clips from different regions, the boundaries seem to blur. Is this just a dialect variation between Owerri and Onitsha standards, or am I missing a rhythmic nuance in how vowels shift in connected speech? I've been using Chickytutor.com to test my own output, but I'm struggling to tell if I'm successfully applying the harmony rules or just guessing.