Problems with the trackball on BlackBerry Tour phones have led to return rates near 50 percent, an analyst at TownHall Investment Research said.
Users are complaining about the issue in a number of online support forums.
They say that they must roll the trackball repeatedly for it to move the cursor on the screen only minimally.
One user said he was on his fourth Tour, after returning the previous three due to problems with the trackball.
Another has posted instructions for how to take apart the phone to try to fix the problem.
Nearly 50 percent of Tours that Sprint sells are being returned, analyst David Eller at TownHall Investment Research wrote in the report.
While the trackball problem is the primary reason for the returns, some customers are complaining about the sensitivity of the touch screen, he said.
High return rates of the Tour for Verizon are an even bigger problem because that operator relies on BlackBerry for the bulk of its smartphones, Eller wrote.
But new phones from Motorola and Palm might change the mix at Verizon.
"Verizon is angry about this recurring trackball problem and is telling its retailers to expect strong support for the new Motorola phone," Eller wrote.
Around Aug. 10, people began reporting in the forums that Research In Motion and Verizon said a firmware fix would be issued in 30 days and would resolve the problem.
Eller expects the issue to put "additional pressure" on RIM's financial results because the high return rates will hurt margins.
"These issues amplify our concerns about RIM's ability to sustain margins while entering the consumer market," he wrote.
Research In Motion did not reply to a request for comment in time for publication. Neither Verizon nor Sprint replied to a request for comment.