WHAT CAN YOU DO when a financial institution won’t accept an essential estate planning document? Kristen McDermott of Fryeburg, Maine, needed an answer. McDermott, 58, a professional gardener, told me she had power of attorney (POA) for her mother, Leslie Kremer, 85, who has dementia. That POA, granted by Kremer 14 years ago and giving McDermott legal authority to manage her mother’s finances, had been honored by Kremer’s bank and former employer. But two financial companies—Vanguard and USAA—wouldn’t let McDermott act for her mother. She had called, supplied documents, called again—and gotten nowhere. “I am at my wit’s end,” she wrote to me.

McDermott’s email shook me. I have power of attorney for my mother, and like McDermott’s, it was executed by a capable attorney. To have it not honored in a crisis would be awful.

First, let’s clarify: Powers of attorney aren’t all the same. A general POA allows a designated person to handle all financial matters on your behalf; a special or limited POA is for a specific purpose, like having someone sign for you at a real estate closing. A durable POA is in force at all times; a nondurable POA ends if you become incapacitated or disabled. And a so-called springing POA goes into effect only if you become incapacitated.

McDermott had a general, durable POA, so she thought she was set.

Vanguard thought otherwise. “We have our own, very specific POA paperwork,” says Lauren Wybar, a senior financial planner at Vanguard. How often are customers surprised to hear they need Vanguard’s special forms? “All the time,” Wybar says.

Vanguard’s version of a POA is called the Vanguard Agent Authorization form, which the company says was designed to be valid in the face of state-by-state variations in POA laws. Presumably, if Kremer had found it among the various forms on Vanguard’s site, filled it out and submitted it prior to the onset of her dementia, McDermott would now be able to access her mother’s account. But—the exact reasons why are still unclear to me—Vanguard made McDermott send in not only a clearer copy of the POA she’d previously submitted, but also a new set of documents: the company’s Agent Certification for Incapacitated Person form and a letter from Kremer’s doctor attesting to her condition.

The problem was different at USAA, which works only with military families. Although USAA would accept McDermott’s POA, it wouldn’t discuss her mother with her because McDermott wasn’t a USAA member. A free membership solved that.

Unfortunately, POA roadblocks are common, says John T. Midgett, president of the National Association of Estate Planners & Councils. “POAs are not given the due deference they deserve,” he says. One alternative is to set up a revocable trust containing your assets. Getting a lawyer to set one up can be costly, though, and doing it yourself runs the risk of getting the details wrong. If you stick with POAs, here are some suggestions.

• Plan ahead. Put your POA on record while you’re well so you can find out whether an institution will reject it, suggests Utah-based estate planning attorney David York. (I think firms need to do a better job of telling you what they’ll accept.)

• Be transparent. “A legal document is only good if someone knows that it exists,” York says. Let loved ones know where you keep copies.

• Consolidate your finances. Because different companies have different rules for POAs, the fewer you have to worry about, the better.

Regrettably, McDermott’s problems were fixed only after I called public relations departments at USAA and Vanguard. “I am fine with the process being rigorous, but it shouldn’t be such a mystery,” she says. “I don’t know if I would ever have figured it out on my own.”