Fitbit has denied claims, made by scores of its users, that a software update it issued in December 2023 has left many devices unusable.
More than 100 users have taken to official forums to say their Charge 5 devices now do not work properly.
One customer told the BBC the update had rendered theirs "useless".
"We're still investigating this issue, but can confirm it is not due to the recent firmware update," a spokesperson for Google, which owns Fitbit, said.
The spokesperson did not offer any alternative explanation for the flood of problems being reported, the most common of which is very short battery life.
"Users should continue to update their devices to the latest firmware and contact Fitbit Customer service at help.fitbit.com if they encounter any issues," they added.
Users on Fitbit's own forums however are adamant the software change is to blame, with some Charge 5 users urging against installing the update, and describing how their devices no longer work properly, if at all.
"Basically, it's useless now, the battery's dead," Dean, in Essex - who did not want his full name made public - told the BBC, as he explained his problems with his Charge 5.
He said previously his device was "working really well" and was "easily" able to last seven days per charge - and said the thought the software update was to blame.
"I don't really see why hundreds of other people would be having the same problem after installing the update if it wasn't."
Fitbit - which was founded in California - helped pioneer the fitness tracker market. It launched its first device in 2009, and has sold more than 120 million units worldwide since then.
It was bought by Google in 2019 for $2.1bn (£1.66bn).
But the wearable tech market has been challenged in recent years by the increasing capabilities of smartphones.
Earlier this month, Google laid off hundreds of staff at Fitbit, with co-founders James Park and Eric Friedman reportedly among those to leave.
Dean said the issue arose in his device immediately after updating it.
"I noticed the battery started discharging immediately - it wouldn't hold its charge," he said.
"The battery was completely dead. I contacted customer services for the online chat, and all they would do is talk me through factory-resetting the device, which wouldn't help.
"Then I logged on to the forums and asked for help there and found that other people were having the problem and it was completely being ignored by Fitbit."
Fitbit's forums are full of similar complaints, some made only this week.
One person said their Charge 5 went "from perfect to non-functional overnight", while another said in a post made on Christmas Day that the update caused their device to be "completely bricked".
Others have criticised the "frustrating" customer service, which offered "no solution other than to buy a new Fitbit".
One customer has demanded Google "be more accountable", saying they intend to switch to a device provided by a rival firm.
"Everyone who has bought a Fitbit Charge 5 anywhere should be able to return these units without question and receive an upgrade to keep faith," they said.
"The company is big enough to absorb the cost."
Additional reporting by Kris Bramwell.