This happens because of the following reasons:

CSP directives present on brand website 

On the website, do Right Click → Inspect → Console. You might see some errors like thes

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These ‘Content Security Policies Directives’ issues arise because the current URL and/or other relevant URLs have not been whitelisted by the brand. Based on the environment on which the client is on, they will need to whitelist the corresponding URLs (Eg. prod/prod2/prod3); please reach out to your SM for more details.

Issue with URL whitelisting in builder

Current URL and/or other relevant URLs have not been whitelisted/added in the LiveChat builder. You need to ensure you whitelist them in this section of LiveChat builder as shown below