Date: 201909
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Weather elements / visibility: Thunderstorm
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: B737-900
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Time Pressure; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Weather; Airspace Structure; Procedure
Primary problem: Procedure

Narrative: I was working Sectors X/Y combined. There was a line of weather spanning from ZZZ VOR northeast bound to ZZZ1 VOR. There were some holes to get some traffic thru; but we had to go into a single flow for ZZZ and ZZZ1. We were asked to route ZZZ2 overflights around on various routes to avoid the weather as well. We got a few ZZZ and ZZZ1's thru initially after being in and out of the hold. There ended up being too many ZZZ and ZZZ1's side by side and weather routing into a single flow got to become too cumbersome. Sectors Z/A had too many deviations; and said they couldn't take anymore. I setup to go into the hold again to slow everything down for them. They said they could take out 1 at a time; but I just needed to get aircraft into the hold to come up with a better plan. We could not hold for the ZZZ's because of weather. I established the ZZZ hold at ZZZ2 VOR with right hand turns and X mile legs. I held ZZZ's at odd altitudes; and ZZZ1's at even altitudes with X mile legs (published holding at ZZZZZ; with left turns). Since the majority of the airplanes had already held several times before; there was more queries and information they were asking to make their plans better with their fuel constraints. This caused a little extra frequency congestion. I began to pull out advantageous aircraft to put on headings northbound for Sector A to work around the weather. I began to transition the aircraft descending to move everyone along sooner. Aircraft X was giving holding at ZZZZZ. He asked to do right turns instead of left because of the weather. I told him unable because of the restricted area; and that I could move his holding pattern farther to the south. I turned Aircraft X left to a 240 heading to bring him farther south. Shortly thereafter; I then saw an opportunity to get this aircraft in so I tried to turn him to a 360 heading to go thru the hole in the weather aircraft were going through. He was unable; so I told him to keep flying the 240 heading. Then I continued descending aircraft. I descended Aircraft X to FL 340. I believed that is what he read back. Then; I had Aircraft Y; I descended to FL350; and I guess I didn't quite realize the holding pattern he was making. I had giving him holding southwest of ZZZ2 VOR on his inbound radial (he came from area 5) so his turns were a bit different than the others that came from Area X. I issued a few more clearances and conflict alert went off. I didn't realize until later that the Aircraft X ended up leveling off at FL350 and wasn't descending to FL340. Another aircraft was tying up the frequency. As soon as I could; I turned the Aircraft Y right to a 270 heading and the Aircraft X I turned left to a 090 heading. It was too late; and the aircraft came within a few miles of each other at the same altitude. This line of weather spanned across [Sectors] Z/A most of the night. There was some holes to work some traffic through; but most of the night it appeared that traffic was still being run into the weather without a plan. There were a few times that we were told to route airplanes into the weather for their re-routes; and it made no sense. ZZZ3 was running back door [routing]; and Supervisor came over and told us that TMU said we could run the ZZZ3 over ZZZZZ1 SIE. This was around XA00- ish; and Sectors X and Y were split. ZZZ1 has just gone into the hold; so Sector X just got busy holding ZZZ1's. We told the Supervisor that we had taking point-out's on the ZZZ3's and that we didn't have any; and that he should tell Area X about the re-route to ZZZ3 for any of them that were farther to the south. Supervisor then came back after going to check with someone; and told us that we had the 2 that ZZZ3's they were talking about. We told him again; that we weren't talking to them; and even if we were; they are just about out of our airspace and established on the back door routing. Why would we turn them over ZZZZZ1 SIE; try to get them down through the holding pattern; and on a routethat still went into the weather. We then called Sector B and asked if they were taking ZZZ3 ZZZZZ1 SIE; they said no because of weather. I don't understand why TMU said to route aircraft into weather. Earlier than that; we were in the hold for ZZZ4 (for the same line of weather that caused a problem ALL EVENING SHIFT). Then we were told to route the ZZZ4's over ZZZZZ2; then over to the JIMMS arrival-into the weather. At this time; the ZZZZZ3 arrival over [fix] was clear on the backside of the weather. There just seemed to be no plan all night long; and there was a lot of extra coordination; volume and chaos that wasn't needed.  I hope this evening's traffic flows are reviewed and we can gain some lessons learned out of this.




Synopsis: Center Controller reported while in holding; an air carrier did not descend to assigned altitude resulting in an airborne conflict.