By Dan Frosch And Robin Sidel DENVER—Two months after the U.S. government tried to smooth the way for sellers of legalized marijuana to get banking services, many pot entrepreneurs still can't open a basic checking account.The Treasury Department and Justice Department in February issued guidance that was meant to reassure banks that they wouldn't run afoul of federal law—under which marijuana remains illegal—by working with pot businesses in states that have legalized the drug.But lenders have taken little comfort in the guidelines, saying they don't go far enough to guarantee that banks won't face legal repercussions, and add burdensome new requirements that they screen customers for marijuana ties.As a result, banks have become even more uneasy about accepting marijuana money, pushing state-licensed pot businesses deeper into a financial netherworld."Most banks are trying to avoid this and mitigate it if they inadvertently find a customer with a marijuana-related business," said Chuck Johnston, president of North Valley Bank in Thornton, Colo., which won't accept customers with marijuana ties.Bank of America Corp. and J.P. Morgan Chase & Co. are among the large banks continuing to steer clear of the issue, bank representatives said. The security company Brink's won't provide armored transport for the marijuana industry because the drug remains illegal under federal law, a spokesman said.Pot stores increasingly find themselves in situations other businesses couldn't fathom, such as hiring armed escorts to help deliver sales taxes to government offices in wads of cash."What I find most frustrating is that I pay an enormous amount of taxes and licensing and other fees," said Morgan Carr, co-owner of the Wellspring Collective pot shop in Denver. Mr. Carr said he recently lost his bank account for the seventh time because the bank he was using became uncomfortable with his cash deposits.This month, Mr. Carr hired a private security firm to haul his cash to a vault in a discreet location because he still can't get a bank to take his business. "The money I am using to pay those taxes is the exact same money that I can't even deposit into my own account," he added.Federal officials declined to discuss the impact of the guidance. In a March speech, Jennifer Shasky Calvery, director of the Treasury Department's Financial Crimes Enforcement Network, said she was "encouraged" the agency had received "dozens" of suspicious-activity reports from institutions banking with marijuana businesses."From our perspective, the guidance is having the intended effect. It is facilitating access to financial services for marijuana-related businesses, while ensuring that their activity is transparent," she said. Twenty-one states and the District of Columbia have legalized medical marijuana, while Colorado and Washington state also allow pot sales for recreational purposes to anyone 21 or older. Colorado, which began allowing recreational marijuana sales in January and has nearly 500 medical and recreational pot shops, collected more than $6.1 million in marijuana taxes the first two months of this year.Business was expected to see a significant bump this month because of Sunday's annual homage to pot smoking known as "4/20 Day" and related events.Colorado officials say the situation not only raises safety concerns—marijuana stores handle large sums of cash, making them vulnerable to robberies—but makes tracking tax payments extraordinarily burdensome. "We're incredibly concerned," said Andrew Freedman, Colorado's director of marijuana coordination. "As long as it is a cash industry, we can't sufficiently regulate their money."Even after the guidelines, Colorado Springs State Bank turns away prospective customers nearly every day because they are tied to marijuana activities, said John Whitten, the bank's market president. "There are legitimate marijuana businesses out there that could be good bank customers, but unfortunately, we have to decline," he said. With most banking options still closed, Colorado store owners are scrambling for new ways to secure their cash. One Denver-area company operated by military and law-enforcement veterans, Blue Line Protection Group, now specializes in shipping cash from pot stores to several vaults around Colorado by armored trucks. The company also transports marijuana."For many of us who were in law enforcement, it's very surreal now that it's legal," said Dan Sullivan, Blue Line's vice president of marketing. Mr. Sullivan previously worked for Colorado's Jefferson County Sheriff's Office. Unsure if the guidelines will be clarified soon, others continue to devise their own methods, sometimes surreptitiously.At Good Chemistry, a sleek marijuana shop near downtown Denver, owner Matthew Huron stocks an ATM with cash. Whenever a customer makes a withdrawal, the machine transfers money from the customer's account into Mr. Huron's account.Having already lost 10 bank accounts, Mr. Huron also uses cash to pay every conceivable bill. He even paid a contractor $1 million in cash to build a warehouse for his business."What really bothers me is that we are working so hard to be good corporate citizens," he said. "And yet they stop everything at the bank. It's irrational." Write to Dan Frosch at dan.frosch@wsj.com and Robin Sidel at robin.sidel@wsj.com