If certain levels of activity are not logged every month customers are charged extra for the device.
The Apple Watch usually costs £369 in the UK.
The firm said a small number of customers were affected and some had synced their devices incorrectly.
Vitality declined to give figures of how many had taken up the offer.
Under the deal, customers who do not submit enough activity points per month have to pay an extra sum of up to £12.50 every time they fall behind.
The contract lasts for two years.
Anna Holt, from Winchester, told the BBC she and her partner had both taken up the offer, and while his data had synced, hers had not.
She was advised to email screenshots of her activity data to the customer service team, she said.
"It defeats the object," she said. "There seem to be a lot of others complaining about it online."
Overnight, her device suddenly started syncing, she said.
There is no issue with the Apple product itself.
The Vitality UK app, which has a number of features for customers alongside tracking the Apple Watch data, has a large number of poor reviews on the app store and the majority of the 667 ratings it has received so far give it one star, the lowest score.
One reviewer complained in November that only steps and not running data were syncing while another said on 10 January that steps were only being recorded as zero.
"I have to constantly email customer service to get them to update it manually," they wrote.
Another described the app as "well-intentioned but muddled, frustrating, constantly changing and usually not working".
Vitality has been contacted for comment.