In a year when digital sales are soaring, several big-name chains have reported issues with their online shopping systems. J. Crew reported on Twitter late Friday morning that it was experiencing “some technical difficulties” as buyers flocked to its site for 50% off everything deals. Lululemon Athletica Inc. said on Facebook late Thanksgiving that it was “working hard to get the dot com running smoothly again.”

“Our North American website experienced some minor disruption in service which the team quickly mitigated to restore our guest experience. We are managing closely throughout this week and into Cyber Monday,” a Lululemon spokeswoman said in an email Friday. “We sincerely apologize to any of our guests impacted by this issue.”

A J. Crew spokesman didn’t immediately reply to requests for comment.

Even Walmart Inc., the world’s biggest retailer, reported issues amid higher than expected web traffic. The big-box store — which saw some of the biggest online-sales increases in the week leading up to Thanksgiving — reported sluggishness before the holidays even began.

“Due to extremely high demand for our Black Friday deals Wednesday night, our site experienced some delays shortly after our event began online,” Walmart spokesman Danit Marquardt said in a statement. “Many customers were still able to check out smoothly, and we were able to quickly fix the issue.”

According to Adobe Analytics, as of 10 a.m. Friday, $643 million has been spent online, up about 28% year on year. Digital spending is on track to exceed $6.4 billion on Friday — maybe even surpassing last year’s Cyber Monday totals, it said.

Mobile shopping represents a growing share of e-commerce this season. That’s because consumers are getting more comfortable making big purchases with their phones and major retailers are investing in their mobile apps, according to Lexi Sydow, a senior analyst for mobile insights provider App Annie.