Some of the possible causes are:
- Zimlet Disabled: After deploying the ZeXtras Chat Zimlet you need to enable it for any COS/user that must use this feature. Enabling the ZeXtras Chat Zimlet is done through the "Zimlet" section of a COS or account.
- Zimlet Disabled by the user: according to your Zimbra's settings, users might be allowed to disable zimlets.
- Missing Server: The ZeXtras Chat server is not enabled.
- Delegated Access: By design, the ZeXtras Chat Zimlet is not loaded when accessing a mailbox through the "View Mail" feature.
- Account Type: By design, the ZeXtras Chat Zimlet is not loaded for System Resource and Calendar Resource accounts.
- Browser Cache: Sometimes the browser's cache interferes with the loading of the zimlet, especially after an update. Try accessing the Zimbra Web Client in dev mode (adding ?dev=1 after the url, e.g.: https://mail.domain.com/?dev=1)
- Server Cache: Caching issues might be also caused by the Zimbra cache. To clear this cache, run `zmprov fc zimlet` as the zimbra user on your server.
- Permissions: It has been reported that sometimes wrong file permissions can be set for the zimlet files. Should this be the case, manually setting the correct permissions or running the `zmfixperms` Zimbra CLI utility should solve the issue.
If none of this applies, please open a support ticket mentioning all information found in section 3.1 and 3.10 of the "How to report a ZeXtras Suite issue" guide.