Deputy team principal Claire Williams said the problem at Spa two weeks ago had been caused by a combination of unfamiliarity with a new system and the failure of backup measures.
"We'd changed the way we laid our tyres out," Williams told reporters at the Italian Grand Prix.
"Normally we had a vertical structure and then we changed it to a horizontal (one) and we just weren't prepared enough for the new system we had in place.
"Equally, we have a 'buddy' system whereby no-one is solely responsible for such a big job but unfortunately that failed. So it was a variety of factors...but everyone makes mistakes so there is no blame.
"We will stick with the new system, we've just got to practise it a bit more."
Williams carried out a full review of their pitstop procedures after the incident two weeks ago robbed the Finnish driver of the chance to fight for a podium finish. Television replays had focussed on one mechanic.
Bottas, who had started in third place, was handed a drive-through penalty after the team gave him three soft tyres and a medium at his first stop -- an error evident to television viewers given that the tyre walls are colour-coded.
He finished ninth.
(Reporting by Alan Baldwin, editing by Justin Palmer)
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