light a fire under

病気を患う母に送るはずだったお金が送られずに、ATMに"食べられて"しまったそうです。お金は戻ってきたそうですが、詐欺師がいるために返金手続きは大変な事が多いそうです。
The Desk: Where's the refund? Machine ate cash meant for ailing mom

Consumers owed a refund on items bought with a credit card can contact the card issuer to dispute the charge. Banks vary on the process, but most typically want proof that consumers have attempted to contact the problem company to work it out on their own. That can be as simple as a list of dates you've left messages with the company to a thick pile of email exchanges.

Consumers also can lodge complaints with the Oregon attorney general's office (877-877-9392), which typically will send a letter to the company involved asking what's taking so long. Be clear in your complaint and offer many details, including why the refund is necessary, the date you first attempted to get your money back and whom you've worked with at the company.

Often, a letter from the state is all that's needed to light a fire under the refund process.

And, of course, there's always The Desk, which will do its part to speed up the process.
http://blog.oregonlive.com/complaintdesk/2012/12/the_desk_wheres_the_refund_mac.html#incart_river

light a fire under O で「<米略式>（人）をせかせてやる気にさせる」（『ジーニアス英和辞典　第４版』大修館）と定義されています。Wiktionaryには、"(idiomatic) To motivate or encourage (someone) to start sooner or move faster. "と書いてあり、語源についても記述されていました。"The origin of the phrase came from when chimney sweeps were scared to go up the chimneys, the fire would be lit under them in order to motivate them to climb to the top."
少しおっかない語源ですが、おかげで印象に残って覚えやすい気がします。(Astroriver)