Here are the possible solutions for you
https://www.spjeff.com/2018/02/01/fixed-unable-to-search-item-by-id-your-search-returned-no-results/
https://stackoverflow.com/questions/42931447/sharepoint-online-list-items-not-appearing-in-the-search-results
https://sharepoint.stackexchange.com/questions/90873/sharepoint-2013-find-an-item-search-box-does-not-return-results
https://sharepoint.stackexchange.com/questions/232268/indexed-sharepoint-list-item-not-being-shown-in-search-results
Please remember to click Mark as Answer on the answer if it helps you