Pat Symonds has revealed that changes to the nose design for 2015 caused a deficit in performance for Williams, something they have worked to overcome.
Although this year's cars will resemble last season's given the largely unchanged rules, there have been major changes to the regulations governing the design of the nose.
In an attempt to do away with ugly noses, the F1 teams agreed to a redesign of the nose.
That redesign has been a "slight headache" for Williams, according to chief technical officer Symonds.
"The change in regulations offered us a slight headache," he told Autosport.
"The new front bulkhead and nose geometry had much more of an impact than we had initially anticipated and the effect on the aero was profound.
"Although the aerodynamics of the  car were impressive there is always room for improvement, particularly as we handle the new 2015 nose regulations.
"The team have worked hard on pulling back the deficit these regulations have made for us."