If a custom action is not showing under the actions menu even when it has been enabled in ActionGrid Setup, take a look at your developer console:
Chrome - Chrome Menu -> More Tools -> Developer Tools -> Console or hit the F12 key
FireFox - FireFox Menu -> Developer -> Browser Console or hit Ctrl + Shift + k
Edge and IE - Edge/IE Menu -> F12 Developer Tools or hit the F12 key
You might see the following error if Enable Content Sniffing protection is selected under the Content Sniffing protection section in Salesforce Setup: Refused to execute script from 'https://www.abc.com/resource/1225399...enametoinclude' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled.
When a user creates an ActionGrid custom action, a custom MIME type is automatically associated with it which is not supported in Salesforce security because the MIME type is not standard.
To fix the issue, go into Salesforce and select Setup -> Security Controls -> Session Settings and then uncheck the Enable Content Sniffing protection checkbox under the Content Sniffing protection section.