Despite Apple’s iOS 8 getting solid reviews, its early adoption rate has been slower than that of its two predecessors -- iOS 7 and iOS 6 -- according to a report from online ad network Chitika. The new report comes at a time when users are complaining about problems in upgrading their devices to the latest firmware.

According to Chitika, the adoption rate of iOS 8 during its first 24 hours of availability in the U.S. and Canada appears to be significantly lower than what was reported for the first day after the release of the iOS 7 and iOS 6. For iOS 8 usage rates, the firm examined a sample of millions of U.S. and Canadian iOS-based online ad impressions from within its network, generated between Sept. 17 and Sept. 18.

“While reviews of the new OS version have been generally positive, initial adoption of iOS 8 has been remarkably more tepid than the last two iOS iterations -- iOS 7 and iOS 6,” Chitika said in a report released on Thursday.

According to the report, iOS 8 was running on about 7.3 percent of iOS devices 24 hours after it was released. In comparison, iOS 7 was installed on 18.2 percent of devices after the first day of its availability while iOS 6 was running on 14.8 percent of devices within 24 hours of its release.

Adoption-Comparison-iOS-8 The early adoption rate of Apple’s latest iOS 8 has been slower than the last two version of the mobile operating system. Photo: Chitika

“The more muted reception of iOS 8 in these early stages is likely due to several confounding factors,” Chitika said, in the report. “Perhaps most importantly, the pool of iPhones eligible to upgrade to iOS 8 is limited to iPhone 5S, 5C, 5, and 4S models, and many outlets have suggested that iPhone 4S users should not upgrade due to subsequent performance and functionality issues.”

Other firms, such as Tapjoy and Mixpanel, also found that iOS 8 adoption rate is much slower than that of iOS 7, Re/code reported.

Meanwhile, many users also complained of issues while downloading iOS 8, which requires up to 5.8 GB of available storage on the device if downloaded over-the-air. Users without the required storage on their devices have to delete or move memory-gouging apps off their device to download the update.

Many users took to Twitter to express their annoyance over the glitch, causing the subject to trend even higher than the Scotland referendum at one point on the micro-blogging site, BBC News reported.

“This update would be great... If you didn't have to delete half of the stuff on your phone just to install it,” one user tweeted.

Another user made fun of the issue, tweeting: “So Apple put the #U2 album on everybody's phone and then tell them they don't have enough space for the #iOS8 upgrade.”

While iOS 7 introduced a new interface to the platform, iOS 8 is more incremental in its improvements, and introduces features such as the ability to add widgets, extensions to share information between apps, and interactive notifications.