Hi @Messam 

I am afraid that the search function on SharePoint is not effected in any way by Nintex, you will need to investigate what columns on your list are indexed and if they are included in the site search or not. 

There can also be a number of issues at play including issues with the search service or platform its self in which case contacting your IT’s SharePoint Server admin team or Microsoft Support might also be a good course of action. 

Jake