You should be able to find out the reason why the estimate not showing by going to the sub-task and click on *Admin* - located at the same level of workflow transition.
After that, try and click on *Where is my field*. From there, you can type in *Time tracking* and proceed. JIRA should provide a brief explanation for the reason and how you can do to fix it.
Also do make sure that the 'Time Tracking' field is enabled for your project or the specific issue type (sub-task).
Finally take note of this too: https://jira.atlassian.com/browse/GHS-9167
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