Twelve months on from its disastrous start to testing after car build delays, Williams enjoyed a strong first week of running in Barcelona as the FW43 hit the track without any major car issues.
But the team suffered a setback on Wednesday morning when an oil system issue forced Nicholas Latifi to stop on track, causing a red flag.
Speaking on Wednesday afternoon following Latifi's issue, deputy team principal Claire Williams expressed her frustration over the recurring engine problems that had limited running.
"We had some engine problems which have been a little frustrating," Williams said.
"We're on our third engine issue now, which has obviously cost us quite a lot of track time, which is unfortunate.
"I suppose it is what can happen in testing, and at least it's not our issue that something has gone wrong with the car.
"Apart from that, it's gone well. We're covering the run programme as we can with the limited mileage that we've got. I would be happier if we had some more miles under our belt I suppose."
Williams has been forced forced into two engine changes through the four days of testing so far, having also suffered setbacks last Thursday and Friday.
Claire Williams confirmed the problems were being investigated by Mercedes, the team's power unit supplier.
"There were some other issues last week. The one on Friday, [Mercedes is] still investigating, so you should talk to them about that," Williams said.
"I don't think that they've found the problem. I think there was an issue with the MGU-H on the Thursday night one.
"There was a sensor issue on Friday morning that obviously was our issue. That put us off the track for probably half an hour, something like that. That's the only issue that we've had."
A Mercedes spokesman told Motorsport.com: "The issue that Williams encountered today is different to the one they experienced last week.
"The first assessment points towards an oil pressure issue. The PU is now back on its way to Brixworth where we will have the chance to take a more thorough look at it."
Williams was able to get back out on track for the afternoon session on Wednesday following the engine issue, with George Russell taking over from Latifi.