One of the perks of paying $99/year to be an Amazon Prime member is the free two-day standard shipping on millions of items sold on the e-tailers’ site. But according to a new survey, being a Prime member didn’t always prove to be beneficial when it came to on-time deliveries last holiday season.

Reuters reports that a joint survey with market research firm Ipsos found there could be an increasing strain on the logistics network that powers Amazon Prime’s two-day shipping operation after a number of consumers were dissatisfied with holiday delivery outcomes.

According to the survey, nearly 10% of the 1,700 Amazon Prime shoppers who chose the two-day shipping option from November 1 and December 31 say they didn’t receive their packages on time.

Analysts say that while customer satisfaction with Amazon Prime as a whole remains high – 96% – the new survey findings reflect the complexity and high cost of shipping orders from distribution centers to consumers’ homes.

If shipping problems continue, satisfaction rates for Amazon could erode, Hayley Silver, a vice president at e-commerce researcher Bizrate Insights, tells Reuters.

“Some customers will remain Prime members because they love the other aspects Prime offers them,” she said, referring to services like streaming video. “Some may not because on-time delivery was the most important thing to them.”

Not surprisingly, Amazon officials don’t agree with Reuters’ findings, calling the figures “very suspect.”

“Our internal data shows significantly better results,” a spokeswoman said. “But any miss is an error and we continue to work very hard to ensure fast, reliable and accurate delivery for customers.”

Because this is the first time Reuters has conducted the survey it did not provide any information regarding Amazon’s past delivery performance.

Reuters suggests that consumers’ dissatisfaction with the late deliveries may simply be a downside to Prime’s increasing popularity, noting that the e-tailer has been exploring ways to speed up shipping and taking more control of the process.

A former employee says that one of the company’s chief concerns is that it doesn’t have control over the entire delivery from warehouse to consumer.

Currently, the company uses local and regional delivery companies to handle the so-called “last mile” – shipping from the distribution center to a consumers’ home.

However, the use of outside delivery companies could change in the future. Amazon UK previously outsourced delivery, but eventually created its own delivery networks after consumers complained about packages arriving late.

Exclusive-Survey: Trouble for Amazon Prime’s two-day deliveries? [Reuters]

Want more consumer news? Visit our parent organization, Consumer Reports, for the latest on scams, recalls, and other consumer issues.