The problem they have is that drones have too small a return for the proximity detonation to work reliably. So they've compensated for that by making the system more sensitive (which is normally unwanted, because the missile encountering chaff dispensed by a jet or cruise missile could be set off before actually intercepting the intended target in the chase aspect).

So now the missiles go off prematurely when the target has a suitably different radar return, and that's when they come close enough to the target for the proximity detonation to work. Command guidance is generally kinda shite for this sort of work - if you want to intercept targets smaller than an actual aircraft, you want an independent seeker on your missile for maximum precision in the terminal phase of flight. We'd probably see similarly bad performance by the system against HARM spam.