A Center Controller and the Controller in Charge reported an aircraft did not respond to an assigned altitude or the Controller missed the readback and aircraft flew into confliction with descending traffic. The Center Controller stated wearing a mask caused the communication difficulties.
Synopsis
A Center Controller and the Controller in Charge reported an aircraft did not respond to an assigned altitude or the Controller missed the readback and aircraft flew into confliction with descending traffic. The Center Controller stated wearing a mask caused the communication difficulties.
Narrative
I initially climbed aircraft Y to 35000 ft. (their requested final); but realized; at the time; they weren't climbing that well and I had some traffic that needed descending into ZZZ. I issued aircraft Y direct routing to pull them to the north and issued an amended altitude of 25000 ft. for the aircraft X ZZZ arrivals; who I proceeded to descend to 26000 ft. I called traffic and received no response from aircraft Y. Aircraft X and the first ZZZ began flashing which does happen on occasion when aircraft are climbing and descending towards each other but they were tail to tail and I didn't think anything of it. Aircraft Y's next altitude showed 25500 ft. I issued a 30 degree left turn and stop climb to aircraft Y and went to aircraft X and issued a 30 degree left turn and stop descent. Aircraft X stopped their descent and made the turn while aircraft Y just turned and continued climbing and extra 1500 to 2000 ft. through aircraft X. When I went to issue a brasher warning to aircraft Y it took two calls to get a response. I was partially distracted as we were discussing our lack of staffing and trying to develop a plan with Traffic Management Unit on how to implement Traffic Management Initiatives. Ultimately I missed the read back of the altitude; but I feel like the pilots of aircraft Y were also not engaged or paying attention. Almost every time I made a transmission to them they either didn't respond or missed part of it. Remove the masks while working traffic. I had said; and have meant to; file reports that since Day 0 after masks being implemented again. I and others have noticed a significant increase in the number of issues with communication. Where I would experience 2 to 5 instances a shift of aircraft not responding; asking for clarification or clearances again; or giving bad read backs; I have been experiencing 3 to 10+ instances a session of these issues since implementation of the masks again. There have been multiple other controllers that have complained about similar issues since the mask mandate's return.
Second reporter narrative
I was Controller in Command looking at staffing for the day as well as a previous radio issue with another sector. I heard from across the room the XXX [sector] controller exclaim something; and the XXY controller sitting right next to him) suggest that he tells the Aircraft X to stop descent. XXX said to Aircraft Y 'stop your climb' and turned them 30 degrees left. Then he told Aircraft X to stop their descent and turn 30 degrees left. While I heard this; I quick looked Sector XXX on the See All Scope. I saw Aircraft Y continue to climb after he was told to stop; and Aircraft X stop their descent. Once the aircraft were safe; XXX sent them both back on course and recovered well. I was not able to grab a handset to listen in quickly; so I only heard the controller's side of the conversation. Later; he told me that he is sure he issued 25000 ft. to Aircraft Y; but he was not sure if the altitude was read back correctly. I have no recommendations.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.