You might have noticed the handy "Save & Build" option in App Center Build Configuration.
Exhibit A
It does more than just help you trigger the build immediately after Saving/Updating changes.
Generally, your app build configuration is analysed when you open the build configuration dialog. So when you have made changes to your repository which don't get picked up by the build automatically, you might need to go to the configuration page again and trigger a "Save & Build" from there.
One such example is when you add new Build Scripts to your repository. Since the build scripts are only analyzed when configuring your branch, you'll need to perform another analysis to re-index your repository tree and save the results.
If you simply made changes to an existing build script which was also already used in a branch configuration, you can simply push your changes to the script.
We're well aware of this limitation and are working on improving re-indexing Build Scripts when changed.
Until then, please bear with us and do a manual "Save & Build".
We'll update this post once this is improved :)