Williams' 2019 car is not yet in Barcelona.
Dead last in 2018, hopes were high that the once-great British team would begin its recovery this year.
But a filming day scheduled for last Saturday was cancelled, and then Williams announced it would not be joining the other nine teams for the start of official winter testing on Monday.
Claire Williams said the team needs "more time".
Then, she announced that Tuesday's running has been cancelled as well, with Wednesday now being the "earliest" day for the FW42's debut.
A source in Barcelona, where Williams' motor home, team members and even drivers are present, said the most likely reason for the delay is external suppliers.
There are also rumours that a design flaw means several parts need to be redesigned.
Robert Kubica was in the paddock on Monday and later trained on his bike.
"The FW42 is not in Barcelona," our source reported. "The car that is here is last year's, with some new parts including the front and rear wings."
However, it is believed Williams has no plans to actually run that car.
A source said that if Williams' new car does arrive in Barcelona, it will be some time on Wednesday, which could mean a third consecutive affected day of testing.
The team confirmed that test driver Nicholas Latifi, who is the son of new McLaren co-owner Michael Latifi, will no longer test this week in Barcelona.
Williams had planned media activities including interviews with Claire Williams and Paddy Lowe for Monday and Tuesday, but they were also cancelled.
A team member said in Barcelona: "We can't comment."