Adam, In your case It is your server configuration …As mentioned before. SLP is working. SLP from loading.
And as far as Google tag manager, yes we know that, but any JSON errors or other errors can stop . The PHP JavaScript page header is a critical part of a working Store Locator Plus installation. Since it loads very late in the page creation process it is susceptible to failures from almost anything else you may be loading on the page or your WordPress site
Look at the screenshot and the error that is showing google tags built into your index.
Also, do you have referrers? Do you have just a browser key. Please read how to restrict your key. Currently it is not being blocked or restricted because I can clearly see it. I am guessing you have it in wrong spot or set up incorrectly for that customer
https://www.storelocatorplus.com/protecting-your-google-api-keys/
or if you do not restrict just add browser key
https://www.storelocatorplus.com/wordpress-store-locator-plus-google-api-key/