In searchhead cluster with six machines, only one SH machine is not giving results for a particular app.
What do you get on the GUI for the search. Do you find any error on the screen?
Is the search head able to do any searches but the search in question? Check the job inspector .
What is the app? What is it supposed to be doing? Is it enabled on all SHs? What are the expected results? Have you checked the logs?