flash in the pan
An effort or person that promises great success but fails. For example, His second novel proved to be a flash in the pan, or We had high hopes for the new director, but she was a flash in the pan. This metaphoric term alludes to the 17th-century flintlock musket, which could be fired only when the flash of the priming powder in the lockpan ignited the charge in the bore. When it failed to ignite, there was only a flash in the pan and the gun did not shoot.