You change a setting, the screen reloads, and the old value is back. GTM Kit now tells you why in most cases. Here is what each message means.
GTM Kit rejected the value
If a value is one GTM Kit cannot accept, the screen names the setting and the reason, and nothing is written. Correct the named field and save again. Before version 2.19 a rejected save reported success and showed your previous settings back, so the change appeared to vanish rather than to fail.
The save did not stick
GTM Kit reads back what it wrote. If the two differ, it tells you rather than reporting success.
On a site with a persistent object cache, that cache is the most likely cause: the new value was written, and a stale copy is what gets read. Flush the object cache from your caching plugin or your host’s control panel, then save again. Redis, Memcached and their WordPress plugins all behave this way when the cache is not invalidated on write.
Without a persistent object cache, the database most likely did not keep the change. Ask your host to check the database error log, then save again.
The settings screen cannot reach your site
Go to Tools → Site Health. GTM Kit checks whether its own settings interface answers, and shows you the error it received when it does not. A critical result here means something is blocking or holding those requests: a security plugin, a firewall rule, a reverse proxy, or authentication in front of the site.
The check tests the path from your server to itself, which is not the same path your browser takes. A pass there with a failure in your browser points at something between your browser and the site rather than at the site itself.
A value you did not type
Browsers and password managers sometimes fill fields on the settings screen. From version 2.19 GTM Kit tells them not to. On earlier versions a filled-in value could be saved without you typing it and reappear after you removed it.
Still stuck
When you have an open support ticket, share your system data from the Support page in GTM Kit. It records whether your site uses a persistent object cache, which is the first thing worth knowing here. With GTM Kit Premium, if sending fails, you can copy or download the system data and email it instead.