When something is wrong

A string stays in the source language

Look for it under Translations → Strings first. If it is not there, WPGlotty never saw it: it is either excluded, inside an element marked translate="no", or produced by JavaScript after the page was rendered.

If it is there but untranslated, it is queued or it failed. The status screen says which.

Nothing is being translated at all

Three checks, in order: is the licence active, is there a provider key, and is WordPress cron running. The third is the usual answer on sites where cron has been disabled and replaced by a system cron that was never actually set up.

The translation is right but reads badly

That is tone, not accuracy, and it is fixed with the tone of voice setting and the glossary rather than with a better model. Set both, retranslate that language, and correct the remaining handful by clicking them on the page.

Visitors see the old text

A page cache. WPGlotty clears the ones it recognises; if yours is at the server level it usually cannot be reached from PHP. Clear it by hand once to confirm that is the cause, then wire it up with the purge filters.

The licence says it cannot be reached

Nothing happens. The plugin keeps its last answer and carries on translating. If it persists for more than a day, check whether the site can reach wpglotty.com at all: a firewall that blocks outgoing HTTPS will also be blocking your provider.