Within the first four months of its release, the Apple Pay adoption rate (those linking any payment card to Apple Pay) surged to 11% percent of all credit cardholders (15% among smartphone owners), according to a new report from Phoenix Marketing International.

Since the first four months of Apple Pay's availability, the adoption rate has climbed to 14 percent, revealing a slowdown in the growth rate. Higher adoption rates are shown among millennials and Gen X'ers and, currently, these two groups account for 92 percent of Apple Pay users.

Phoenix, a global marketing services firm, released new data as part of its ongoing consumer tracking report on Apple Pay.

Some of the findings' highlights include:

• Two-thirds of Apple Pay users have specifically asked if the merchant accepts Apple Pay.
• Nearly half of Apple Pay users have expected Apple Pay to be accepted – only to find out that the specific store visited did not in fact accept Apple Pay.
• A majority of Apple Pay users continue to encounter problems at the point-of-sale, including terminals that take too long to use, terminals that don’t work properly and cashiers unable to help.
• A lack of customer support is another reason why adoption has been slowing. Apple Pay users tend to be confused as to whom they should contact when an error, such as a double charge occurs. Forty percent of users needing support are calling Apple, but really do not know who to call. Ultimately, Apple and its partners have to be ready to handle issues regarding customer support.

Phoenix said in a press release the findings also revealed the importance of customer support in maximizing the Apple Pay opportunity. 

"An influx of inbound inquiries from Millennial and Gen X cardholders creates an opportunity for card issuers to engage customers and differentiate the bank brand,” said Greg Weed, Director of Card Research at Phoenix Marketing International. "Issuers are faced with new challenges to influence wallet position and differentiate benefits and services beyond those that are automatically delivered through the Apple Pay wallet. Future growth still depends on marketer’s ability to differentiate the merits of digital vs. plastic card transactions."