Williams felt it was quick enough in the dry to get a good result in the British Grand Prix, but felt it lost too much time in the wet part of the race to recover into the points.
Rubens Barrichello and Pastor Maldonado finished only 13th and 14th, despite the latter qualifying seventh.
"It was quite difficult for us today especially with the light rain shortly before the start," said Maldonado. "Our car set-up was geared towards a dry race so we were compromised from the outset. Towards the end, our performance was better but by that stage it was too late. Unfortunately it wasn't the best weekend for us."
Barrichello lost several positions at the start and agreed that the car had been a handful in the wet.
"We didn't have the best of starts and fell back quite a bit," he said. "Strangely, the car didn't feel good in the wet and it was a struggle to keep it on the track. It was also difficult to keep up with the guys in front of us. We had hoped for a better result at our home grand prix."
Technical director Sam Michael was confident that Williams would have been quick enough in a totally dry race.
"Our pace on the intermediate tire wasn't good enough today," he said. "Compared to some of the guys in the top 10, our lap times on the dry tire were respectable, but we just lost too much time in the opening laps to be able to make up any ground. We ran different strategies on the two cars to attempt to do something different to recover but it wasn't meant to be today."