Small maneuverable targets at high speed, poor optics (new ship, lol), lack of a tracking system for the gun?/not able to be used at night?/unable to track at close range?
Also AK-74 with mk.1 eyeball =/= Hitrole-NT gyro-stabilized 12,7mm with a fire control computer and FLIR.
Helicopters seem to have a better record at destroying naval drones. Close in the russian weapons systems are of the Spray & Pray variety.