(CNN)-As serious supply chain problems continue to exist around the world, it is difficult for stores to quickly put infant formula back on the shelves.

Walgreens, a drugstore chain with more than 9,000 outlets in the United States, said that while its suppliers are struggling to quickly meet orders, the national demand for infant formula is increasing.

"[This] may cause temporary and isolated shortages," said Walgreens spokesperson Emily Hartwig-Meckstein.

Walgreens said it regularly posts product availability updates on its website throughout the day.

"We are replenishing the store as quickly as possible and continue to work hard with our supplier partners to best meet the needs of our customers," Hartwig-Mekstan said, adding that the retailer is meeting the demand.

It is not always easy for parents to switch from one brand of formula to another, because some babies may have intolerance to the ingredients of a particular brand.

According to data from market research firm IRI, which tracks retailer point-of-sale data, unit sales of infant formula have fallen by 1.7% year-on-year. The unit sales of liquid concentrated infant formula milk powder decreased by 18.3% year-on-year. IRI did not explain why the numbers were down-or so obvious for concentrated brands. Sales in these two categories have fallen in the past few years.

However, when you can’t get it or keep it on store shelves, it’s difficult to sell a lot of formula.

Reckitt Benckiser, who owns Mead Johnson Nutrition (the manufacturer of Enfamil infant formula milk powder), said the company has ample supply, "to ensure that all babies get the nutrients they need."

The members of the US Infant Nutrition Council, including formula manufacturers Abbott Nutrition, Gerber Products, Perrigo Nutritionals and Reckitt, said its members are actively working to mitigate the impact of current challenges affecting global supply chains, transportation and distribution systems.

"This includes working with distributors, retailers and state agencies to ensure the supply and continuous access of infant formula in innovative ways. Parents and caregivers should ensure that they obtain infant formula from safe and reliable sources," the organization Said in an email to CNN Business.

Gerber (Nestlé brand), Perrigo, CVS and Target did not respond to requests for comment.

One of the most stolen items in stores is infant formula, and since it is also one of the most frequently stolen products in retail, shortages in stores have become complicated.

A 2020 survey of 61 retailers of the American Retail Federation, the industry’s largest trade organization, showed that organized retail theft increased by nearly 60% from 2015, and every $1 billion in sales cost stores an average of $719,548 .

Organized retail crime involves a group of people who steal entire shelves of products such as cosmetics, infant formula or popular jeans — not for their own personal consumption — but to resell them for profit.

The items most likely to be stolen in stores include designer clothes, washing powder, razors, deodorants and infant formula. According to NRF's 2020 Organized Retail Crime Survey, infant formula accounted for 13% of the most stolen items in stores. Using formulas, theft can be triggered by personal needs, inability to pay, and resale.

In August, a couple in Shelby County, Alabama, admitted to selling stolen infant formula worth more than $300,000 online.

"Infant formula has become a top priority for thieves because it is easy to resell quickly. We are increasingly seeing infant formula resold online," said Jason Brewer, spokesperson for the Retail Industry Leaders Association, a trade organization. "Given the increasing number of thefts experienced by retailers, [theft] is definitely a factor [leading to the shortage]."

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