There are a few reasons why the height of a template iframe might be set to zero, thus rendering it invisible:
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Piano uses the height of the contents within a template to calculate the template height. If you interfere with that process (for example, by using CSS to set the height of the content to 100% or any other percent value), the result will be a template height of zero because Piano's templates only recognize fixed-height settings.
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Another common reason is forgetting to put a closing tag in a template, which can break the entire layout. If your template isn't appearing, double-check that you've closed all tags.