Random #NAME? errors

I’m using this plugin to display the latest financial update for my church. The source spreadsheet contains the report data on the first sheet, which is calculated from data on other sheets. Most cells in the report sheet make use of custom functions to calculate their values.

On my main development machine, everything displays as expected on my WordPress site. When I viewed the site from another machine, however, every calculated value which uses a custom function displayed #NAME?. Upon loading the Google spreadsheet from the machine in question, everything started displaying as expected, without me making any changes to the Google sheet. This is weird, because I presume the plugin makes a server-side request to Google, and thus is not affected by the browser. Yet what other explanation is there?

Now, I’m on another machine. For testing, I will not be opening the Google sheet on this machine. I keep getting the same #NAME? errors as before. No tweaks I’ve made to the WordPress site have changed anything. Changing the cache settings has no effect.

However, the site has always worked just fine from my phone. Plus, I got an email from a site user who clearly was able to read the data without #NAME? errors.

Why are these errors coming up sometimes?

The page I need help with: [log in to see the link]