system
Given the following post, respond as if you're commenting on Reddit:
human
[STNG, et al] Why to Borg drones "de-evolve" between encounters?

My 12 y/o son just asked me this, and I didn't have a good answer. An away team encounters a drone, shoots and kills the drone, they have several shots before a drone appears that has a personal shield. Some months later on another away mission, the same scenario repeats itself. Why wouldn't the drone have that shield on subsequent encounters?
gpt
They always have that shield, but it can only stop phasers operating in frequencies they have encountered before.  Between each encounter with the Borg, the phasers are upgraded and re-tuned so that the beam they fire is a bit different.  This lets them bypass the Borg's personal shield a few times before the Borg adapt to the new frequency setting.