Most commuters feel pretty worn out by the time they leave work and board a SEPTA bus home.

Equally worn out by the end of the day, apparently, are some of SEPTA's new Key card readers.

The transportation authority is tracking a small but disquieting number of card reader failures on buses since the beginning of the year. The readers seem to be working in the mornings, but by day's end, problems consistently appear. The issues aren't limited to any one route or geographic area.

SEPTA first noticed the failures in January, according to data the authority provided. That month, there were failures on 0.24 percent of card transactions over the course of the day, a minuscule number. The rate has crept upward, though, as transactions through SEPTA Key have climbed from about 148,000 a month in January to 4.3 million in September. In that most recent month, SEPTA logged card reader failures on 2.17 percent of the daily card transactions.

SEPTA has no estimate yet of fare revenue lost because of card reader failures but said it is likely small.

The transit agency has a theory about what's driving a lot of those failures, and it's linked to the popularity of Key cards. About 400,000 cards are in circulation now, and every time one is used, the system checks it against a list of all the cards in use to confirm its validity. That's not really a problem in subway stations, but buses, always on the move, depend on a WiFi connection. About 3 percent of the time, a transaction happens in a dead zone. The buses' card readers should be capable of running that validation process when disconnected from the system, but they're getting bogged down searching the lengthy list, causing failures.

"We're going to do a software update and fix the problem we think is the problem," said Sam Sulaiman, SEPTA's director of maintenance and engineering for revenue.

The solution, SEPTA hopes, will be to have the system check transactions against cards that are known to be invalid — those that have been reported lost or stolen, officials said. It's a much shorter list and would be less taxing on the card reader when a bus is in a dead zone. The software improvement was to begin testing Friday and was expected to be rolled out across the system in a few weeks, he said.

While SEPTA has given no formal direction to bus drivers, people with Key cards that aren't being recognized are allowed to ride.

"Certainly if somebody has a card and the system's not reading it through no fault of their own, they're not going to be asked to exit the bus," said Andrew Busch, a SEPTA spokesman.

Glitches like this aren't a surprise on a system as complicated as SEPTA Key, said David Schuff, professor of management information systems at Temple University's Fox School of Business. The burdens placed on a system by wide usage can be hard to predict, he said.

"Often, when you really need to scale something, it changes the way these technology systems work," Schuff said.

SEPTA is still learning about the quirks of its new system. One surprising problem, said Rich Burnfield, SEPTA's deputy general manager and treasurer, is that Washington, D.C.'s equivalent of the Key card will freeze a SEPTA card reader if it comes in close proximity to it.

SEPTA has also been dealing with user confusion involving the card dispensing kiosks. Kiosks with redesigned operating screens are in place at 10 locations along SEPTA's system, with more improvements planned.

SEPTA intends to phase out all its other fare tools and rely entirely on SEPTA Key, though the exact time frame for that isn't clear. Right now, the smart card works on every mode of travel except the Regional Rail, and an early adopters program is planned to begin for commuter rail in December.