Regarding your issue, what version of PI coresight are you using? Can you make sure that the application pool and/or user mapping has the necessary access to the PI Server to at least read the PI tags? Most common reason for this symptom is that the security settings are not properly configured such that the Coresight does not have the necessary permissions to search the PI tags. Please check on the PI Server to check if there is any messages indicating such errors for the PI Coresight.
It would be good to contact OSIsoft TechSupport to have the TechSupport engineers to have a look at your setup directly instead of going through the forum for help.