Valtteri Bottas concedes Williams struggled to make the gains it had hoped for in the second in-season Formula 1 test at Silverstone.
Williams tried three different front wings over the course of the 98 laps covered by Bottas on Wednesday, as he set the sixth-fastest time.
The double-decker rear-wing trialled by Felipe Massa in the first test at Barcelona in mid-May also appeared late in the day.
Given its difficulties of late, failing to score a point in a race for the first time this year in the British Grand Prix, Williams had hoped the test would result in considerable strides.
"We ran three front wings, all from this year, ones from the beginning [of the season] and the very latest ones, and we found small differences," Bottas told Autosport.
"We are honestly still struggling to see big differences as we would like.
"We were predicting bigger gains, but at the moment we're struggling a little bit to put it directly as downforce and laptime.
"We've found some results which could benefit us, not in the very near future, but later on."
Williams ran the rear wing - which is outside of the current regulations - for a day-and-a-half at the Circuit de Catalunya, but only in the final hour at Silverstone.
"It's adding downforce," Bottas said.
"The main thing is trying to fine tune the balance a bit better, and how much we gain with 'x' amount of downforce, especially in cornering conditions because that is the weakness we have at the moment - cornering grip.
"So we are trying to see what it does, if we can make the downforce in the corner a bit better."
Bottas also confirmed that aero updates to the FW38 - "which have been planned for a long time" - are lined up for the final two races before the summer break in Hungary and Germany.
"The overall feeling was definitely we improved some things a little bit," he said of the test.
"Small steps at a time, there's still a long season to go. It was definitely an important test for us.
"We'll look at the data, and especially the long runs we did to try and use those things to improve our race pace, but aero-wise the stuff we did today is more for next year."