A Center Trainee Controller reported a military aircraft encountered an electrical malfunction and descended out of their assigned airspace in the vicinity of a military UAS flying near the airspace. The Controller attempted to vector the UAS to ensure separation but the UAS did not respond.

Narrative

Aircraft X was operating in the ZZZ ATC assigned airspace (ATCAA) and was assigned a block [altitude] of 25;000 to 41;000 ft. (top of block is approximate). Aircraft Y was operating in Locations X; Y; & Z at an altitude of 24;000 ft. At the time of this incident; Aircraft Y was located near the southwest corner of ZZZ (ATCAA). Aircraft X [requested priority handling] due to an electrical malfunction and requested an emergency descent\return to ZZZ1. An altitude and direct route was requested with ZZZ1 Approach and the aircraft was issued the descent. In order to ensure positive separation; the R side told Aircraft Y to turn to a heading of 090 to make way for the descending emergency aircraft. Aircraft Y did not reply to the vector. The controller made a subsequent attempt at vectoring Aircraft Y. Again; no response. The controller made multiple additional attempts to contact Aircraft Y with no success. As the emergency aircraft was descending; the position conflict alert activated. The emergency aircraft was able to descend rapidly enough and there was no loss of separation between the 2 aircraft. In addition to the ongoing emergency; the fact that Aircraft Y wasn't responding to the controllers instructions significantly increased the complexity and potential danger of the impending situation.This problem of Aircraft Y not responding is a repeated problem with these aircraft on a constant basis. It is unclear if this is an equipment issue with the aircraft or a pilot training issue. After the situation was resolved; the pilot of Aircraft Y finally responded and stated that he didn't hear any of the clearances issued. There were no known frequency issues at the time. The emergency aircraft landed safely without any further incident.The R side controller; a trainee; handled the situation properly given the situation at hand. The pilots of Aircraft Y need to be aware that monitoring the frequency is imperative to safe operation while they fly their missions.