The AK safety acts by putting a steel lug on top of the right side of the trigger.
If the trigger failed to return all the way forward, the safety wouldn't be able to be applied.
A possibility is some fouling or debris fell into the trigger assembly. When you cleared the rifle, whatever it is fell out or moved.
Clean your rifle's trigger assembly and check for debris.
Also, make sure the "legs" of the hammer spring are properly resting on top of the rear of the trigger.
The twisted wire hammer spring has hooks on the ends that hook over the raised edges of the trigger at the rear.
If one side of the spring is bent, broken, or slipped off, it may prevent trigger reset or block the safety.