The understanding of unfamiliar or accented speech is a cognitive function. It’s possible you’re hearing enough actual sound to hear what’s being said, but cognitively you don’t associate those sounds with the way you recognise specific phonemes etc.
We get it around here when people retire to this area from England; all of a sudden their speech recognition declines. Their hearing hasn’t substantially worsened per se, it’s just the shift in the accentuation, dialect and even the speed of delivery is different.
Eventually, with the right help and sufficient mental plasticity, things improve. Difficult to resolve in the short term though.