Did you do them in full range of motion (starting from dead hang) and slow eccentric motion on the way down? Not all workouts which are equally exhausting are optimal for strength.
Were you completely resting between sessions? How’s your sleep?
I did do the full range of motion and started from dead hang; sometimes I did them slowly, but often just at a regular speed, though definitely never fast. I did not usually do multiple sets so usually was fully rested between sessions. IIRC was sleeping fairly standard at the time ~8 hrs. I don't carry a lot of muscle / strength naturally which may or may not be relevant; e.g. my untrained grip strength is very low, only slightly higher than my wife's.
Were you completely resting between sessions? How’s your sleep?