By Kristina Dougherty

PHILADELPHIA (CBS) — As more and more people do their holiday shopping online, there are reports that UPS and FedEx are struggling to keep up with the demand.

According to the Wall Street Journal, the volumes have “blown past” expectations, delaying the delivery of the some of the millions of orders shoppers have placed since Thanksgiving.

FedEx and UPS handle millions of packages on their busiest days, so even a small discrepancy can mean tens or hundreds of thousands of customers receive their shipments late, the WSJ reports.

They say UPS had been expecting to handle over 700 million packages, a 14 percent increase from last year, while FedEx predicted a 10 percent surge.

Both companies said daily demand during the peak holiday season can reach double their average daily shipment volume.

A spokeswoman for UPS tells the WSJ the carrier has faced various challenges this year, which have held up a number of deliveries.

“A small percentage of packages have experienced some delay related to weather, wildfires or some operational challenges.”

The spokeswoman also said UPS is moving personnel to busier locations to help out.

However, a logistics consultancy expert tells WSJ, he’s not sure how much good it will do.

“If their supply chain is already starting to get clogged, the issue could compound on itself and we could experience a situation where people aren’t getting their packages for Christmas.”

“Because e-commerce is growing so explosively, it is hard to know how much volume to anticipate,” he said.