Valtteri Bottas has admitted that Williams is struggling to convert updated parts into lap time gains, after trialling three front wings during testing at Silverstone.
Bottas registered 98 laps at the second in-season test, which took place after the British Grand Prix, and recorded the sixth quickest time.
Williams has endured a difficult string of races, racking up only 11 points from visits to Baku, Austria and Britain, leaving the team under pressure from Force India in the battle for fourth in the Constructors' Championship.
"We ran three, all from this year, from the beginning and the very latest ones, and we found small differences," Bottas said, referring to Williams' trial of the front wings.
"We honestly still struggle to see big differences as we would like to see.
"We were predicting bigger gains, but at the moment we're struggling a little bit to put it directly into downforce and lap time."
Bottas is nonetheless hopeful that Williams can make progress when Formula 1 returns to action at next week's Hungarian Grand Prix.
"We have some updates lined up for Hungary and Germany aero-wise already which have been planned for a long time," he explained.