Williams technical director Pat Symonds says his team's development has been "remarkably stagnant" compared to rivals Force India this year.
After finishing third in the last two constructors' championship, Williams finds itself in a battle for fourth place with Force India this year. It currently sits fifth in the standings, 10 points behind Force India, and has dropped 25 points to its rival in the last five races.
After the opening five races of the season, Williams was 51 points ahead of Force India, but a major upgrade for the British-based Indian team at the Spanish Grand Prix started to turn the tables.
"I think we have been remarkably stagnant this year and other teams, particularly Force India, have moved on," he said. "Force India did most of it in one upgrade, which is quite unusual these days because you tend to trickle things in race by race, but at Barcelona they made a pretty major step forward. It may not have been quite as apparent in Barcelona -- I think it took them a little bit time to understand it and best utilise it.
"We have also introduced upgrades and we have improved the car -- if we went back to Barcelona now we would be a bit quicker, but the delta wouldn't be as much as Force India. They have done an exceptional job, but I think our development, which has been a strong point in 2014 and 2015, has let us down this year."
Symonds was reluctant to blame Williams' early switch to its 2017 car and instead admitted the team's development plan had not delivered the results he expected.
"Certainly the facilities and infrastructure are exactly the same as it was [in 2014 and 2015], and most of the people are the same and we know they are capable of doing a good job.
"We did make quite an early switch this year to next year's car, but that's not all of it because some of the things we concentrated on at the start of the year to become our developments right through the year simply didn't work as well as we expected. We have a target of what we expect to get out of the windtunnel in terms of [downforce] points per run and this year it was not as good as it was last year."