Citi Bike expanded to the Upper West Side with great fanfare about a year ago, but the operator admittedly struggles to keep the neighborhood docks full of bicycles.

The problem has left commuters fuming when they approach a docking site to find it empty and have to scramble to find another way to get to work.

“Unless you’re going to work at 6 or 7 a.m., you’re not getting a bike,” said Ian Steiner, 28, who lives on the Upper West Side and works in Midtown.

He tried to grab a bike at the dock at West 88th Street and West End Avenue at about 10:30 a.m. on Wednesday, but found that all the docks were empty.

Citi Bike expanded its coverage north to West 110th Street last year, but has not perfected a system to transfer enough bikes to empty docks — called rebalancing — to cater to the demand.

“Rebalancing is one of the biggest challenges of any bike share system, especially in a city like New York, where residents don’t all work a traditional 9-5 schedule,” said Citi Bike spokeswoman Dani Simons.

“And though there is a central business district, it’s a huge one, and people work in a variety of other neighborhoods as well.”

Part of the problem is that commuters who live on the Upper West Side tend to ride Citi Bikes to work, but then use other methods to come home rather than bringing bikes back into the neighborhood, according Madeline Kaye, another spokeswoman for the bike-share program.

Kaye said the problem of where riders leave bikes has been the most severe on the Upper West Side, although it has affected other neighborhoods as well.

Streams of disappointed commuters check the Citi Bike app or show up to empty docks each rush hour and wonder how they are going to get to work.

“I’m actually late for work because this a–hole got the bike,” said one angry rider who got to a dock just as someone else rolled off with the last bike.

Rick, a 29-year-old law student who lives on the Upper West Side, says Citi Bike needs to do a better job.

“I don’t know why they can’t just bring more back-up,” he said.

The company said it has 150 rebalancers who use work vans and box trucks to move bikes from one part of the city to another.

And it has more than 8,000 “bike angels,” customers who help stock empty docks in exchange for incentives such as membership extensions and day passes.

But as ridership has grown to about 60,000 cyclists a day, it’s just never enough.

“We continue to use a broad array of tools and expand our efforts to ensure there are bikes where and when they’re needed,” said Simons.