Hello,
This may be caused by the dispatch and storage fractions, so please check that those are correctly assigned on the Thermal Storage input page.
There is an issue with the dispatch controller algorithm that causes it to fail to dispatch TES when the sun is down under some conditions, especially when the power cycle is not continuously running. If you would like us to review your file to see what might be causing the problem, please [url=mailto:sam.support@nrel.gov?subject=Power%20Tower%20Startup%20from%20TES t=_self]email it to me[/url].
Also, for the next version of SAM (planned for release in January 2016), the power tower model will include an improved controller with optional dispatch optimization.
Best regards,
Paul.