The Grove-based squad approached the season certain it had taken a step forward compared to 2012, during which its car proved fast enough to win in Spain.
But neither Bottas nor Pastor Maldonado managed to break into Q3 or the points in the Australian or Malaysian Grands Prix.
Maldonado has retired twice, with Bottas's 11th place at Sepang remains the team's best result.
Bottas admits that he was expecting to challenge for stronger results.
"Of course I was hoping to fight for better positions," he said when asked by AUTOSPORT if he was frustrated.
"The goal is to get higher and be consistently in the points.
"It's what the team was hoping for and what I'm hoping for so of course it's a bit disappointing.
"It is how it is now, we just need to get back there.
"We're understanding more all the time. We need more time but we will get there."
Bottas insists he is convinced of the potential of the Williams FW35 despite the lacklustre start to the campaign.
The team recently said it suspected the exhaust area was the biggest weakness of the car.
"It's a different car, [and] some things were better in the  car," Bottas said.
"We have more potential to do better in this car though. It's not easy but we're learning more and more.
"The goal for this year is to improve the position from last year so we need to get working."
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